Reveal is the amount of pleat showing across the back, generally measured at the bottom of the fell. in a well-made and well-pressed pleat that hangs properly the reveal should be of uniform width from the fell to the selvedge. Here is a good example, IMO. Notice how the stripe is centered in the reveal all the way down.

*ok, steve beat me to it. he's the pro anyway*. but i have the pic!