This document discusses Riemann sums, which are used to approximate the definite integral of a function over an interval. A Riemann sum takes the area under a curve and approximates it using rectangles. It does this by dividing the interval into subintervals and using the values of the function at the left or right endpoint of each subinterval to determine the height of each rectangle. The closer the subintervals, the more accurate the Riemann sum approximation will be to the true value of the integral.