Access to some of the content requires a user name and password. This comes from private access of the forum, Patreon, SubscribeStar, or given directly because of contributions to the site or being a frequent commissioner. Sometimes a temporary account is given out just because.
Regardless, to use it, this is where you enter it.
Due to the nature of the current authentication, the account is shared across all subscribers and the password changes only when significant new content is added. You will not be logged out, you'll just lose access to the content and a new login page will be presented. In those cases, get an updated password from the original place (usually the latest post has the password at the bottom) and re-enter it.