Provably fair is a way to prove that a random result was fixed before you bet, not picked after. The site commits to a secret server seed by showing you its hash in advance. You supply a client seed. Each bet adds a nonce, a counter. The site combines the three with a known formula to get the result. Later, the site reveals the server seed, and you can rerun the formula yourself to confirm the result matches.
What does provably fair actually prove?
It proves one narrow thing: the outcome of each roll was determined by the seeds and the nonce, and the site could not swap in a worse result after seeing your bet. If the revealed server seed hashes to the value shown before, and the formula gives the result you saw, then the roll was not tampered with. You can check this with the provably fair verifier.
It does not prove the odds are good. A site can be fully provably fair and still pay out far less than it takes in. The house edge lives in the payout table, not in the random number. A case that costs 10 dollars and holds items worth 8.50 dollars on average is provably fair and still loses you 1.50 dollars per open in the long run. Provably fair says the dice are not loaded. It does not say the game is worth playing.
How do you verify a provably fair result?
Find the fairness page on the site. Note the hashed server seed before you play, and set your own client seed rather than using the default. After the round, or after you rotate seeds, the site shows the unhashed server seed. Hash it and compare to the earlier hash, then run the site's published formula with your client seed and the nonce for that bet. If the numbers match, the roll was fair in the narrow sense.
A few warning signs undo the whole idea. A site that never reveals the server seed, changes the formula without notice, or does not let you set a client seed cannot be checked. And a fair random number attached to a game with a 10 percent edge is still a game where the expected result is a loss. Read the provably fair guide, and see responsible gaming before you play.