This is a cool GeoGebra demonstration designed by the ideas we're using in chapter 5 to define the Definite Integral.
He includes a good description of how to use it. Think about how this relates to the definition of Area on page 365 and the definition of the Definite Integral in page 372. As you move the lower slider to the right you are getting better and better approximations of area as more rectangles are added.
The Definite Integral, Area and Riemann Sums
As always, comment or email me if you have any questions.