I'll explain it a bit more fully:
Add adds the colour value to the background, and that is the colour of the pixel. So, if you have an animation that has a totally black background, that won't change the animation at all if it's in 'add.'
Normal is just that, it changes the pixels to the value of the graphic
Sub subtracts the value, so a completely white circle in sub would make a completely black circle, while black would change nothing, red would remove the red value, leaving blue/green, et cetera.