He, moved by zeal for Ráma’s sake, The hill Maináka thus bespake: “O strong Maináka, heaven’s decree In days of old appointed thee To be the Asurs bar, and keep The rebels in the lowest deep.

In plain words: The Sea, wanting to help for Rama's sake, speaks to the mountain Mainaka. It reminds Mainaka that long ago the gods gave it the job of keeping the Asuras (demon enemies of the gods) trapped deep below.