Book Report is a bookmarklet. It is like a bookmark, but instead of bringing you to a different page when you click it, it runs some code. In this case, the code grabs your sales data from Amazon's servers and displays it for you.
The details of what the code does changes frequently, to keep pace with changes Amazon makes and to add new features. But the goal is always the same: Book Report grabs data from Amazon and lets you analyze it with ease.