The ten incarnations of Vishnu is a Hindu theological concept in Vedic history. Vishnu exists as the Creator and also within every being as our soul, our Supreme. He also enters or descend to our world in the form of Avatar whenever his presence is needed.
Avatar is described by Krishna:
Whenever there is a decline of religion, and a rise of irreligion, I incarnate myself. To protect the good, to destroy the wicked, and to re-establish religious principles, I appear in every age ---( Bhagavad Gita )
The Ten Avatars or Dasha Avatar of Vishnu are important and very interesting as you can see that he incarnates progressively from fish to human form.