Composition determines how a sprite is drawn with respect to the background.
Note: When tested with "Chrome Version 46.0.2490.80 m" on Windows 8.1, the 'composition' setting didn't work as expected. However a test using Chrome on xOS had no problems. Firefox works correctly as well, as does Safari.
Be aware that many composition operations affect the background so that you can make the background disappear altogether. You usually won't want this - but it can make for some interesting effects.