What exactly is meant by 'roll' here? Does it refer to the part where the collar flap is attached to the body of the shirt or the outer edge of the collar flap?
Well, this is hard to put into words. The visible part of the collar is called the "leaf." That is, it is the part the folds down over the band. Imagine that you are looking straight-on at a shirt that is buttoned up with a knotted tie is in place. Let us, for the purposed of this discussion, call the two visible triangles on each side of the tie knot the collar's "blades." (I happen to know that this is not a real term, because Alex Kabbaz told me so. But it may be useful to us.) Now, the blades each have a front edge: the edge that goes from the collar band at the top of the knot down to the point. On a collar with no roll, that edge will be straight. "Roll" means that it will have a gentle curve, sort of like a tilde (~) tilted at a 45 degree angle, but less pronounced.