It tells us, let's say we have some function capital F of x, and it's equal to the definite integral from a, some constant a to x of lowercase f of t dt. The second fundamental theorem of calculus tells us that if our lowercase f, if lowercase f is continuous on the interval from a to x, so I'll write it this way, on the closed interval from a to x, then the derivative of our capital F of x, so capital F prime of x, is just going to be equal to our inner function f evaluated at x instead of t. It's going to become lowercase f of x. Now I know when you first saw this, you thought that, hey, this might be some cryptic thing that you might not use too often. But we're going to see that it's actually very, very useful. And even in the future, and some of you might already know, there's multiple ways to try to think about a definite integral like this, and you'll learn it in the future. But this can be extremely simplifying, especially if you have a hairy definite integral like this. And so this just tells us, hey, look, the derivative with respect to x of all of this business, first we have to check that our inner function, which would be analogous to our lowercase f here, is this continuous on the interval from 19 to x?