Once logged-in, visit My Details in the Members' area to update your profile or change your password.
Your login times out after 15 minutes or so if there is no activity on the website. Keep me logged in automatically logs you back in when needed. Ticking it means that when you come back to the site you won't be challenged for your login details. It's particularly important if you are editing something and take more than 15 minutes (e.g. interrupted by a phone call): when you save your changes, there's no problem if you've ticked the box, but if you haven't, your changes could be lost. It expires after a month or so. Clicking logout cancels the automatic login and forces a login when required.