[powrtoch]

Roughly, a sound that happens during some measure of music, or an arbitrary point during that duration.

Imagine folding a strip of paper exactly in half. Now you have a crease at the middle of it. If you fold it again, you have a couple more creases at the 1/4 and 3/4 points. The above quote is talking about the same thing, but with a length of time instead of a strip of paper. The more folds you have to make in order to put a crease at a certain point (event), the less salient it is.

Hope that's helpful.