Slow rebound usually is done on the rear dampers. The dampers compress normally, but extend more slowly. This results in a more stable handling, especially on rough surfaces, as the damper doesn`t extend abruptly after a fast compression due to the rough surface, thus reducing the "bounciness", prefereably in the rear. It usually makes the rear handle more smoothly and less aggressive.
At least this is what I have read about it. I do not have any first hand experience with slow rebound as of yet, since my Yeah Racing shocks only just arrived in Germany and I am at least a few days away from trying out slow rebound myself.