SLI means you can connect TWO sli approved nvidia graphics cards together for a performance boost. It is really only necessary if you are a hard core gamer. For the average user, one graphics card is more than enough power.
Some would argue that an SLI motherboard also opens up your future upgrade options, as instead of buying a new graphics card when the current one feels dated, you can purchase a second identical card for an upgrade. I however don't agree with this line of thinking. Unless you are a hardcore gamer who is addicted to super-high framerates, a non SLI board will due just fine.