The idea of this is that when we calculate the slope of a line we use two points and find the change in y-values divided by the change in x-values. If we use the same technique for a curved graph the answer isn't exact though, but an estimate. However the close and close the two points are the close the answer gets to the actual slope. is the second y-value and f(x) is the first. As we let delta approach 0, the limit converges with the actual slope at that point.