Server seed
The casino's secret random value ("graine serveur"). Mixed with your client seed and the nonce, it decides your bet results.
The server seed (graine serveur) is what the casino brings to the randomness of every provably fair bet. You don't get to see it while you're using it; it only comes out once you switch to a new seed.
Why you can't see it
If you knew the live server seed, you could work out every result coming up and only bet when you'd win. So while you're playing, all you get is its hash.
Why it has to be locked in
If the casino could swap seeds after seeing your bets, it could pick one that makes you lose. Showing you the SHA-256 hash first blocks that, because nobody can find another seed that gives the same hash.
Your move after rotating
Grab the revealed seed, hash it with SHA-256, and line it up against the hash you were shown before your first bet. Same? Then plug the seed, your client seed and your nonces into a verifier and rebuild your results.
Real-life example
Before your first bet, the hash on screen starts with 9f86d0... After 200 bets you rotate and the seed comes out. Hash it and you get 9f86d0... too. The casino kept its word.