The Background dialog is a tool to define Background for the active selector/class in the active stylesheet.
A background can be either a color or an image. If you select an image for e.g. the BODY background (page background) there are various settings to choose from.
4 different settings are available. "Repeat" is default and will cover the whole background with the selected image. Try out the other settings and see what happens!
You can control the vertical and horizontal position of the background. Defaults are top and left.