You're just going to get a ton of rectangles over there and they're going to become better and better approximations for the actual area. And the actual area under the curve is denoted by the integral from a to b of f of x times dx. And just to make it, you see where this is coming from, or how these notations are close, or at least in my brain, how they're connected, delta x was the distance for each of these, was the width for each of these sections. This right here is delta x, so that is a delta x. This is another delta x, this is another delta x. A reasonable way to conceptualize what dx is, or what a differential is, is what delta x approaches if it becomes infinitely small. So you can view this, you can conceptualize this, and it's not a very rigorous way of thinking about it, it's an infinitely small delta x.