The document discusses integrating a function f that is continuous and greater than or equal to 0 on the real numbers. It states that to justify this with a diagram, one can make a substitution of x=t+c in the integral, so that dx=dt and the limits of integration change from a+c to b+c, or a to b. This allows the integral to be written as the integral of f(t+c) from a to b, with the integration variable replaced from t to x.