It refers to an appearance of Buddha or Bosatsu (Bodhisattva) as Buddha, Bosatsu or Myoo (Atavaka, general of the heavenly cohorts, who was accused of eating human flesh), in order to relieve living things depending on the capability of people.
仏や菩薩が、衆生を救うため、その機根に応じて現れる仏や菩薩、また明王となった姿。