Sessionhash is used by PHP as an alternative to cookies to keep a user logged in if the user has cookies switched off in their browser, or is surfing through a proxy.
You can switch off sessionhash, but this means editing all your templates, and setting cookieuser and sessiohash in the user table to 1 permanently - I have done this with my forum with no ill effects.
If you don't know how to do it, I suggest you keep the sessionhash, as it is there for a good reason.