From Online Manual
Once a user has completed the registration process, they will be able to login to the site for the first time. After that, they will be able to choose whether to stay logged in forever or log out at the end of their visits.
When a user is registered or they might simply have been logged out from a previous visit, They have the choice of the normal login screen, and the quick login options.
The login screen
Since this screen (accessed from the Login option of the main menu) is not only where a user will normally be taken when they are finished with registration, but it is also the first thing they will see in any 'members only' forum.
While the Username and Password fields should be self-explanatory, Minutes to stay logged in simply means the number of minutes the login cookie will last before expiring and requiring them to log in again. It is not recommended to enter a long time here if the computer is shared, but they might prefer to select Always stay logged in if no-one else has access to the computer.
The quick login
Although many forums also have a quick login box, its position on the page may vary considerably from forum to forum. In the SMF default theme, it is in the collapsible section at the top, whereas it is more likely to be at the bottom in themes based on the older YaBB SE software. If it isn't visible and it has not been collapsed, it is not enabled at all or the user is already logged in.
To use the quick login box, the user should enter their username and password, select their session length from the drop-down menu, and click Login. Forever, like Always stay logged in, simply means that the computer should remember the user until they next choose to log out, and not that they will be online all of that time.