Sli

Because SLI means you are using 2 graphics cards (or more), which means that your PC is going to draw a lot of power. So, you see SLI certifications on power supplies because that is the manufacturer's way of advertising that their power supply can provide enough power for a SLI setup.

It's not always true, however, which is why I say it is "advertising" by the power supply manufacturer. For example, you can find many 550W power supplies that are labeled as "SLI compatible" but in reality you would never want to put an SLI setup on a 550W PSU.

+1 for BigMack70

Just like to add that it will have additional supporting wires for powering 2 or more graphics card at a time in SLI configuration.