Rebound is the amount of bounce your hammer has when you let it fall onto the face of the anvil.
This is typically the test done to check if its a good anvil. You can do it with a hammer or even a ball bearing.
Poor rebound indicates a problem with the anvil, usually that it is cast iron which can be confirmed by a lack of ringing when struck. The test could also indicate that the face plate has come loose.
Looks like a fine anvil mate. Fastest way to clean up the face is to start beating hot steel on her!
All the best
Andy