Gnome3 has the annoying habbit of turning off the monitor after a while, even if you are watching a video. Unfortunately, if you go toSystem Settings > Screen
you will be surprised that if offers you the possibility to change the time the system has to be idle for the monitor to be turned off from 1 minute to 1 hour. But what is missing is an option to turn this function off entirely.
Fortunately, it is possible to do that, just not in System Settings and you can either do that from command line or with a GUI.