Trampled upon by the feet of Krishna as they changed position in the dance, the snakes fainted and vomitted froth much blood. Seeing the head and neck of their king thus bruised and blood flowing from this mouth, the females of the snake-king implored the mercy of the Slayer of Madhu.

In plain words: Krishna's dancing feet crush the snakes until they faint and spit up blood. Kaliya's wives see their husband hurt and bleeding, so they beg Krishna, called the Slayer of Madhu, for mercy.