As you can see in this screenshot, a column called "Maturity" is shown on most mining pools' found blocks table. I did my best to find an answer to my own question with keywords such as "monero block maturity" or "cryptonote block maturity", but couldn't find anything.
It seems like a certain number of steps or confirmations needed when a block is found by a pool (I guess it's the same when mining solo), but I can't be sure.
Sub questions: this value obviously goes from x to zero. What's the value of x, and is it always the same? EDIT: Ok, just saw the "Maturity Depth Requirement" in my own screenshot... sorry! Still, this original question remains. ;-)
Thanks a lot!