I forget whawt SLI stands for but basically, it allows the use of 2 graphics processors to run on one system. People basically get 2 video cards and put them in the SLI slots and the motherboard will split the work between both computers making in theory, a graphics system 2x as powerful as their single counterparts.
Two identical video cards work in tandem to process the graphics for the computer. The cards must be identical, there is a bridge or two that connect them so one card knows which portions of the page the other is doing.
It is kind of a workaround hack until the card makers can come up with a dual Graphics Processing Unit with Physics Processing Unit on one card.
It's only necessary when you are a hardcore gamer who wants to squeeze every last frame per second out of your machine.