In Hindu mythology, Lord Vishnu is the deity who protects and preserves the universe.
Whenever the forces of evil threaten to annihilate the world, it is believed that Lord Vishnu incarnates to save it from destruction by overcoming the evil entities.
There are ten incarnations or avatars of Lord Vishnu. The tenth, Kalki, who is supposed to appear in the current Kaliyuga, is yet to appear.