This is a good introduction to the basic concept of interest. We will warn you that it is an older video so Sal's sound and handwriting weren't quite up to snuff then.
Compound Interest Basics
Interest is the basis of modern capital markets. Depending on whether you are lending or borrowing, it can be viewed as a return on an asset (lending) or the cost of capital (borrowing). This tutorial gives an introduction to this fundamental concept, including what it means to compound. It also gives a rule of thumb that might make it easy to do some rough interest calculations in your head.