Ridged things have a raised line or band, like a ridged hillside or a ridged crease in an old person's forehead.
A ridge is a raised row that can be seen or felt, and when something's described as ridged, at least part of it is shaped like a ridge. Raking your garden bed leaves a ridged surface, and pleating a piece of paper makes it ridged even when you unfold it. Both ridge and ridged have an Old English root, hrycg, "back of a man or beast," from the sense of a ridged backbone.