Shingon, 真言 means true word and is the Japanese reading of the Chinese word Zhēnyán, the translation of the Sanskrit term mantra The term Shingon translates to "true word", the meaning of the Sanskrit word mantra differs. "Man" means thinking and "manas" mind. "Tra" can be interpreted as tool. One way on looking on this method is: Mantras are a tool to explore our mind. The Sino-Japanese translation of the Sanskrit term Mantra illuminates this more from the content than from its function, like cosmic speech or language of Buddhas.
Recitation is a wide spread method used in many religions. You may know the recitation of ōm from Yoga practice, the rosary in Christianity or the use of the Subha in Islamic practice. In Buddhism and Hinduism a Mala is used to count the recited mantras. In Japanese, Mala is called Nenju or Juzu, which is carried on the left arm and has typically 108 beets.
Shingon is called Mikkyō 密教, in Japan, which translates to ‶secret teaching″, which is mostly translated as esoteric Buddhism in the west. In context of Buddhism as a whole, Shingon belongs like Tibetan Buddhism to the Diamond vehicle or Vajrayana. It is often also called Tantric or Mantra Buddhism. Vajrayana is a late development of the so called great vehicle, Mahāyāna.
Mahāyāna developed some 2000 years ago in India, by inventing the idea of the Bodhisattva, the enlightened being, who continues practice for the sake of all living beings. Tantric Buddhism started developing at Nalanda University in North India, when the methods and ideas of Yoga Tantra started entering Buddhism. These different schools never understood themselves of being separate from each other or Theravada, orthodox, Buddhism.
Characteristic for Shingon is the teaching of Sokushinjoubutsu - 即身成仏 attainment of enlightenment during this life time. This marks a big difference to the classical Buddhist schools, like Yogācāra (mind-only-school) or Madhyamaka (emptiness-school), where one was practicing for eons to attain enlightenment.
In the 9th century the teachings of Shingon were brought to Japan by a monk known as Kūkai or Kōbō Daishi, often just called Daishi Sama in Japan.
Kūkai 空海 (774–835), was born as Mao into the well known aristocratic Saeki family. According the legend he was born at his family temple Zentsuji in the Sanuki province on Shikoku Island. Today it is called Kagawa-Ken and Zentsuji is a very important Shingon temple on Shikoku island and part of the 88-temple pilgrimage. He was supposed to become an official at the emperor's court. For this purpose he studied Confucianism and Taoism on the university in Nara. He became interested in Buddhism already when he was young and was studying buddhist scriptures as well. With 22 he wrote the Sangō Shiiki 三教指帰, Guide to the Three Teachings, an allegory in which he lets compete Confucianism, Taoism and Buddhism against each other. Already during the time of his study he was initiated into the practice of the the morning star meditation - Gumonjihō 求聞持法.
Shortly after, he left the university and lived for some years as mountain ascetics in the mountains of the Kii peninsula. Not much is known about his activities during this time. At the end of this period he found one text in a temple in Nara, the Mahavairocana Sutra, which he couldn't grasp fully.
Therefore he decided to go to China for further studies. He became a fully ordained monk and received the dharma name Kūkai, air and sea. He was soon able to join a mission to China in 804. Interestingly Saichō, the later founder of the Tendai school, joined the same mission, but a different boat. After a stormy passage he arrived 805 to Changan and met master Huiguo 惠果 (746 – 805) the abbot of the Qinglong Monastery. In a very short time Kūkai received all the necessary transmissions from him and became a master in Shingon Buddhism. After his teacher Huiguo passed away, Kūkai returned back to Japan. In 810 he became abbot of Todaiji-temple in Nara. In the following years he not only established the Shingon school in Japan, but became a known calligrapher, poet and even did engineering by restoring a water reservoir, which still can be seen. In Shikoku. He also helped finishing the Tōji-temple in Kyoto, which was erected when the government moved from Nara to Kyoto.
In 812 he received the right from emperor Saga to establish a center for practice far away from the noisy capital at a place today known as Kōyasan. As Kōbō was also the abbot of the new Tōji temple in Kyoto and adviser to the imperial court, he was traveling quite often between these 2 places. In 831 he became sick and passed away in Koyasan 4 years later. The legend says he's now sitting in eternal Samādhi at Gobyo in Okonoin at Mount Koyasan another legend tells that he's waiting in Tsushita heaven to return back. Posthum he received the honorable name Kōbō Daishi 弘法大師 - big teacher spreading the dharma. Today Kōbō Daishi is venerated by many Japanese like a saint.
The Shingon method is called in Japan Sanmitsu 三密 or the method of three mysterial manifestations of body, speech and mind. Usually a mediation has a fixed form, which the student receives from his teacher during a personal transmission. During the practice the participant enters into Samādhi, a state of concentration, and returns back to the world here and now. The goal is to establish specific aspects into oneself, represented by the central deity of the meditation through unifying with the object of the meditation. Shingon meditation can be also seen as ritual, performed for the people in the audience to help them to overcome certain hindrances in their lives. Especially for this purpose the fire meditation, called Goma 護摩 is known well. You can participate in this during the morning ceremony in our temple.
The Sanskrit term Mudra, In 印 in Japanese means seal. Mudras are hand gestures, like the praying position, Gasshō. Mudras have a long tradition and go back to the times prior of the vedas, like Mantras. Nowadays they are also used in a lot of different traditions and cultures, like in the practice of Yoga Asana. Mudras are seen as "wizard techniques" or a method to direct or to seal energy, we use it to focus our mind or to copy the body posture of a Buddha or a certain deity into our own body. When you see some Buddha- or Bodhisattva figures, they are usually depicted by doing one or more Mudras.
The practice of reciting mantras goes back to the pre-vedic tradition and can be as short as one syllable, like "ōm" or quite long. Very long Mantras are also called Dhārani in the Shingon tradition. Mantras are usually written in Siddham script (Sanskrit letters), but the original pronunciation got lost over the years. In the Shingon school mantras are transmitted from teacher to student with a Japanese pronunciation. Most of the mantras have an outer or exoteric and an inner or esoteric meaning, which can be only experienced through continuos practice. The probably most famous mantra in Japan is "Namu Amida Buttsu", "Homage to Amida Buddha" used by the pure land schools. For the Shingon School the Mantra for Kōbō Daishi: "Namu Daishi Henjo Kongo" 南無大師遍照金剛, "Homage to the great Teacher brilliant shining Diamond" is probably known best.
The Sanskrit term mandala means circle. The two pictures on the left and right side of this text are called Kongokai- (left) and Taizo-Mandala (right). Both Mandalas are playing an important role in Shingon practice. They can be understood in different ways. One way to understand them is like mapping reality from the relative, our, point of view, which is presented in the Taizo or matrix mandala and from the absolute or enlightened point of view in the Kongokai or diamond realm mandala. Both mandalas are not representing two different realities, but two different aspects of it, like the two sides of a coin. Both mandalas contain different deities, Buddhas and Bodhisattvas. Any of these figures can be represented as a figure or as a Sanskrit Siddham letter or root syllable.
Each of these figures represents different qualities of the mind, like the four wisdoms in the Kongokai mandala surrounding the central cosmic wisdom represented by Dainichi Nyorai - Mahavairocana. Therefore mandalas can be also seen as a mind-map.
These figures or their Siddham letters are used during meditation as objects for visualization. If now all three methods of body, speech and mind come together to enable a one-pointedness concentration, we talk about Sanmitsu practice.
There are 2 nice articles in the Wikipedia about Shingon and Kūkai, linked in the text. If you have deeper interest, here is a list of books worth reading: