Rik is heaven, Sâman the sun. This Sâman (sun) rests on that Rik (heaven). Therefore the Sâman is sung as resting on the Rik. Sâ is heaven, ama the sun, and that makes Sâma.

In plain words: The verse is compared to heaven and the song to the sun, which shines in heaven. So the song rests on the verse, as the sun rests in the heavens.