The server seed is the site's half of a provably fair result. The site creates a long random string, keeps it secret, and publishes its hash. A hash is a one-way fingerprint: you can turn the seed into the hash easily, but you cannot turn the hash back into the seed. By showing the hash first, the site commits to the seed without revealing it.
Why is the server seed hashed?
If the site showed the raw seed, a player could predict results. If the site showed nothing, a player could not tell whether the seed was changed after the bet. The hash solves both problems. You cannot predict results from it, and once the seed is revealed you can hash it yourself and confirm it matches what you saw before you played. Any change to the seed, even one character, produces a completely different hash.
The server seed alone does not decide the result. It is combined with your client seed and a nonce that counts your bets. That is what stops the site from choosing a seed that happens to lose for everyone: it does not know your client seed when it commits.
When does the site reveal the server seed?
Most sites keep one server seed active across many bets and reveal it when you rotate to a new seed, or after a set period. Until it is revealed, you cannot verify the rounds it covered, so rotate seeds if you want to check a session. After rotation, the old seed appears in plain text on the site's fairness page, and you can run each past bet through the provably fair verifier.
A hashed server seed proves the roll was locked in. It says nothing about the odds. The house edge on a case, a crash game or a coinflip is set in the payout rules, and a perfectly honest server seed still feeds a game where the expected result is a loss over time. Check the fairness math, and see responsible gaming for limits that help.