The “Honji Suijaku” theory is a key concept in Japanese Buddhism, particularly in the Esoteric Buddhist (or Vajrayana Buddhist) tradition.
The theory articulates a way of understanding the relationship between Buddhas (and bodhisattvas) of Buddhism and the native kami (deities or spirits) of Shinto, which is the indigenous religion of Japan. According to this theory, the Buddhas and bodhisattvas of the Buddhist pantheon are the honji - the true, original form - while the kami are their suijaku - their local, provisional form in Japan.
This concept allowed the two religious systems of Buddhism and Shinto to exist in a syncretic relationship in Japan. It suggested that when a Buddha or bodhisattva wished to save sentient beings in Japan, they would manifest as a kami, suitable to the culture and understanding of the local people.
Essentially, Honji Suijaku is a theory of religious syncretism that allowed Buddhism and Shinto to coexist and mutually reinforce each other in Japanese society. The theory shaped the religious landscape of Japan and played a major role in the development of its unique Buddhist practices and interpretations.
see also Honji suijaku - Wikipedia
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