When you log in, the board sets a cookie in your browser so that you can browse the forums without having to enter your username and password each time. Logging out clears that cookie to ensure nobody else can browse the forums with your account.
To log in, simply click the login link in the board's top menu. To log out, click the logout link in the same place. In the event you cannot log out, clearing cookies in your browser will have the same effect.