The Rik (veda) is this earth, the Sâman (veda) is fire. This Sâman (fire) rests on that Rik (earth). Therefore the Sâman is sung as resting on the Rik. Sâ is this earth, ama is fire, and that makes Sâma.

In plain words: The Rig Veda's verses are like the earth and the Sama Veda's songs are like fire, and fire rests on the earth. In the same way, the song rests on the verse. The word Sama is split into Sa, the earth, and ama, fire.