What is meant by the term "selah" in Psalms?

It's a term found at the end of a phrase, and generally means: "forever". Ibn Ezra adds, that it has the connotation of truth, similar to "Amen". The Radak adds that it was a musical note, at the end of an idea, symbolizing to the singer to raise his voice (see the root: SL"L, Tehilim 68, 5, to raise). These explanations don't contradict, but rather embellish each other.