Where a is less than or equal to x is less than or equal to b. Or that's just another way of saying that x is in this interval right over here. Now, when you see this, you might say, oh, you know, the definite integral, this has to do with differentiation and antiderivatives and all that. But we don't know that yet. All we know right now is that this is the area under the curve F between a and x. So between a and let's say this right over here, this right over here is x. So f of x is just this area right over here.