Slow rebound = bounces back slowly (so it doesn't turn your bike into a pogo stick/catapult when you land a 20ft drop on 8" travel), fast rebound = bounces back quickly (so your bike doesn't bob slowly & sluggishly every time you ride over a 3" high root, thus wasting energy).

What kind of shock (brand, model) do you have? Rebound adjustment is a feature that varies from one shock/fork to another.