Calculus
The way to do this is to divide the graph into many very small pieces, and then draw very thin rectangles under each piece. As the rectangles become thinner and thinner, the rectangles cover the area underneath the graph better and better. The area of a rectangle is easy to calculate, so we can calculate the total area of all the rectangles. For thinner rectangles, this total area value approaches the area underneath the graph. The final value of the area is called the integral of the function.