HMAC
A hash that uses a secret key. Provably fair games feed the server seed, your client seed and the nonce into HMAC-SHA256.
HMAC is short for hash-based message authentication code. In plain words, it's the standard way to mix a secret key into some text using a hash function. The HMAC-SHA256 version runs on SHA-256 and spits out 32 bytes.
Where it shows up in your bets
In the most popular setup, the casino's server seed is the key. The text being hashed is your client seed, the nonce and a round counter, glued together with colons:
HMAC-SHA256(serverSeed, clientSeed:nonce:round)
You then chop the 32 bytes into 8 blocks of 4. Each block turns into a number between 0 and 1 by reading it as a base-256 fraction: take byte one over 256, add byte two over 65,536, byte three over 16,777,216 and byte four over 4,294,967,296. The game converts those numbers into your roll, drop or multiplier.
Why not a basic hash?
HMAC was made for this exact job: mixing a secret key with public input. It also dodges length-extension problems you can get when you just hash strings stuck together.
Heads up
Not every site copies this format. Some flip key and message, add fields or convert bytes their own way. Always verify with the method that casino documents.