Never heard of Roll's theorem, i'm assuming you're talking about Rolle's theorem.
The theorem states that in a continuous graph on a closed interval, if any two points have the same y value. For example if f(3) = 4 and f(6)=4, then in between those two points there is a value that has a derivative on zero. Meaning that the slope is zero.
It sounds a little confusing but it's really easy, basically take two points on a graph. If anywhere in between those two points you have an x value that has the same y value, then in between those two points you'll have an x value that has a derivate of zero.
mathematically it could be written as
f(a) = f(b)
Then f(c) = 0