You divide the horizontal length under the part of the function you want to evaluate into "n" equal pieces. That is the n on top of the Σ (Greek letter sigma). The (x-x) represents the size of one horizontal segment that is created from dividing the whole by the "n". The f(y) is a y value in an "n" segment. Since the area of a rectangle is "length" × "width", the multiplication of x and f(y) is the area of a rectangle for that part of the graph. The Σ means we add up all of these small rectangles to get an approximation of the area under the segment of a function.