Simply put roof pitch is the slope of your roof. The image is a basic graph that will show you your roof pitch from Flat to Steep that will help you estimate your pitch for the online estimator.
To get into a little more detail, roof pitch is calculated by the number of inches it rises vertically for every 12 inches it extends horizontally. For example, a roof that rises 6 inches for every 12 inches of horizontal run has a 6-in-12 pitch.
If you would like to figure out the exact pitch of your roof you can follow these instructions.