That's our Riemann integral. It's really that right now, before we come up with the conclusion of this video, it really just represents the area under the curve between two endpoints. So this right over here, we can say, is the definite integral from a to x of f of t dt. Now, this right over here is going to be a function of x. And let me make it clear. Where x is in the interval between a and b. This thing right over here is going to be another function of x.