In this context, she is the goddess of wine, who emerged during the Samudra Manthana (churning of the ocean) and chose Varuna as her consort.
She is one of the two chief consorts of Varuna, the other being goddess Gauri.
According to the Puranas, when the Devas and Asuras were churning the ocean, Varuni came out of it and had a pot of alcohol in her hands.
The third goddess is a Matrika found in Matsya Purana.
As per the story, she was created to drink the blood of a demon Andhaka.