Rik is the white light of the sun, Sâman the blue exceeding darkness (in the sun). This Sâman (darkness) rests on that Rik (brightness). Therefore the Sâman is sung as resting on the Rik.

In plain words: The verse is compared to the sun's bright white light, and the song to the deep dark blue inside the sun. The darkness rests on the brightness, just as the song rests on the verse.