The idea of maintenance mode is to prevent any users other than admin to use the site while the maintenance process takes place.
When students/guests/teachers try access a course in maintenance mode, they will obtain a message saying the course is in maintenance mode. A customized message may be added, perhaps stating when the site will be available again or giving the reason for doing maintenance.
If for some reason the webinterface is not available you can also put the Moodle installation in maintenance mode by creating a file called maintenance.html in the folder called 1 in your moodle data folder. Any text in that file will displayed to users trying to access anything other than the frontpage.
- Using Moodle Maintenance mode? forum discussion