Server seed
The casino's secret random value that, together with your client seed and the nonce, decides each bet result.
The server seed is the randomness the casino contributes to a provably fair game. It stays hidden for as long as it is active and is disclosed only once you switch to a new one.
Hidden while in use
Were the active server seed visible, you could compute every coming result and bet only on winners. During play you therefore see nothing but its hash.
Locked by a commitment
If the casino were free to swap the seed after looking at your bets, it could choose one that makes you lose. Publishing the SHA-256 hash in advance rules this out, since no other seed can be found that produces the same hash.
After rotation
Copy the seed the casino discloses, hash it with SHA-256, and check it against the hash displayed before you played. When the two agree, feed the seed into a verifier along with your client seed and nonces to rebuild your results.
Quick illustration
Before you start, the displayed hash begins 9f86d0... You place 200 bets, rotate, and receive the seed. Hashing it produces 9f86d0... as well, so the commitment is confirmed.