Hi mate,
Its basically the amount of rebound you get when pushing the piston all the way up to the shock body then letting go. Do this without the spring on of course. When letting go the piston will push itself back down, i have mine set so it pushes about 25% of the overall length of the shock shaft.
You adjust this by how much you bleed your shocks. The less air/oil you let out the more rebound you will get, and visa versa.
Hope this hasnt confused you
lol.