Varaha was the third incarnation of Lord Vishnu. In Sanskrit Varaha means a boar. Lord Vishnu took Varaha avatar to defeat a Demon named Hiranyaksha. Hiranyakasha was a powerful demon who demanded people to worship him instead of Gods. The people refused him , Hiranyaksha took the earth deep inside the cosmic ocean to teach people on earth a lesson.
Mother earth cried for help to Lord Vishnu. to help her Lord Vishnu took the Varaha Avatar and fought with Hiranyaksha . Lord Vishnu placed Mother Earth on his tusk and fought with him. after a long fight for thousands of years Lord Vishnu killed Hiranyaksh and then he placed Mother earth back at her place in solar system. its even said that lord Vishnu married Mother earth in this epic .