Normally when Jenkins is restarted, it immediately starts trying to schedule jobs to run. If you are taking the server down for maintenance, you may not want everything to start running immediately.
If so, go to Manage Jenkins » Quiet Restart and check the box. When the server is restarted, it will be in the "quieting down" state. An administrator can cancel that state using the regular UI.
You may also wish to uncheck this box after maintenance is complete, so that after subsequent restarts jobs will start running.