In Bitsbox, a fill is either a background image or a pure color. Fills always sit behind all the other objects (stamps, text, etc) on the screen.
Usually you can get what you want by choosing a color or a picture you'd like to see. (See the appendix for a list of valid fills.)
You can also call it with three numbers between 0 (darkest) and 255 (brightest) to mix your own color from red, green, and blue amounts.