Simplest way I can think of:
A single graphics card has to process every single frame by itself (no big deal).
In SLI you have 2 (or more, depending on the setup) cards. Card 1 renders frame 1, at the same time, card 2 renders frame 2, then card 1>frame 3, and card 2> frame 4.
Logically, this would give double performance.
Realistically its ~40-80% depending on the game.