We won’t spend much time in this course studying Riemann sums and the formal definition of the definite integral, but I encourage you to read the relevant parts of the book (Sec 5.1 and the first part of Sec 5.2) and try to understand the concept.
You can play around with the “Visual Calculus” visualization of Riemann sums I showed in class on Monday. There are a number of such visualizations on the web; for example, take a look at Wolfram MathWorld’s Riemann sum page.
Khan Academy has a sequence of 10 short videos on Riemann sums which you may find helpful. Here is the first one: