Provably-fair means you can prove the draw was not tampered with. Not even by us.
Before you play, the house locks a secret server seed and shows you only its hash, the commit.
Every round uses the server seed, your client seed and a counter. The result is deterministic.
At the end, the house reveals the server seed. You check the hash matches the commit and recompute the rounds.
HMAC-SHA256 over server seed, client seed and nonce generates each result. Same seeds, same result, always.