It looks like a fancy term. But all it means is, look, the slightly larger x, I'll evaluate its y-coordinate. Where the curve is at that value of x. So that is going to be, so the change in y is going to be f of x naught plus h. That's just the y-coordinate up here. Minus this y-coordinate over here. So minus f of x naught. And that's our change in, so that equals our change in y.