In plain words
The poet, to the Lord: O you who hold the radiant discus! As a swan you taught the four Vedas [to Brahmā]; besides that, you tore open the beak of the bird [the demon Baka]; you took the bird [Garuḍa] as your banner, and beyond that you ride that noble bird as your mount. Since you are a protector by nature, what is it, that you have loved to lie in the [milky] sea upon the bird's natural enemy [the serpent Ādiśeṣa]?