The client seed is your half of a provably fair result. The site commits to a secret server seed by publishing its hash. You add a client seed, which can be any string you choose. The two seeds and a nonce go through a fixed formula to produce each outcome. Because the site committed to its seed before it knew yours, it could not pick a seed that guarantees you lose.
Why does the client seed matter?
Without a client seed, the site would control the whole input. It could generate thousands of server seeds, find one that produces bad results for the next hundred bets, and commit to that one. The hash would still check out. Your client seed breaks this because the site cannot test seeds against an input it has not seen yet. This only works if you actually set your own seed. Many sites fill in a default client seed for you, and a default the site chose gives back some of that control. Change it to something of your own before you play.
The client seed does not need to be secret. It is fine for it to be visible on your fairness page. Its job is to be unpredictable to the site at the moment the server seed was committed, not to be hidden afterwards.
How do you set and use a client seed?
Open the site's fairness or provably fair settings. You will see the current hashed server seed, a client seed field and a nonce counter. Type in any string, save it, and note the hashed server seed at the same time. When you later want to verify, rotate the server seed so the site reveals the old one, then feed the old server seed, your client seed and each nonce into the provably fair verifier.
Setting a client seed makes a fairness check meaningful, but it changes nothing about the odds. The house edge is set by the payout rules, and a game with an edge loses money for players on average no matter who chose the seeds. The provably fair guide explains the whole chain, and responsible gaming covers limits.