The story of Dakshina Devi begins when yajnas performed by Devas, saints and humans did not yield any results. They approached Brahma Dev for a solution. Brahma Dev meditated on Bhagavan Vishnu and came up with a solution.
From the right side of the body of Goddess Lakshmi appeared a goddess. As that goddess had emerged from right side (dakshina) she was named as Dakshina Devi.
She later married Yagna Deva, the one who presides over all yagnas. A son, named Phala, was born to them.
After this Yagna Deva, Dakshina Devi and Phala bestowed the results of yajnas to people performing them.
After this incident it was mandatory to give Dakshina or alms to all those helped in performing the yajna.