Websites leave what are known as 'cookies' on your computer Mel and these store and gather pieces of information such as log ins, passwords etc.
You can disable cookies by opening the Tools menu, click on Options, then Privacy where the information about the cookies on your computer will be shown. If you decide to disable the cookies you would need to log in every time.
It's easier to simply log out of this website and log in again when necessary if remaining logged in is a problem. Just click on Logout underneath your username at the top of the forum page.