It's basically a measure of how far from the center of the wheel that the wheel is mounted on your car. Or, to simplify, how far out the wheels/tires will stick.
For an explanation that comes from a full-scale car website but is still applicable to our stuff, read this:
http://www.tirerack.com/wheels/tech/...jsp?techid=101
In RC terms, stick with wheels made specifically for your car and you'll be happy.