Normally several eligible playlists share a screen in turn. Priority override stops that.
While its schedule is active, a priority playlist takes the screen exclusively and nothing else plays. When the schedule ends, the other playlists resume on their own.
Turning it on
Open the playlist, go to Advanced, enable Priority Override, and save.
What to use it for
Content that is pointless if it is only seen sometimes. A closure notice, an evacuation message, an announcement that has to land today.
The alternative, deleting or unassigning everything else, is a change somebody has to remember to undo. Priority reverts on its own.
Two priority playlists at once
They do not fight. If more than one priority playlist is active, they play consecutively, sharing the screen between themselves while still excluding everything non-priority.
The mistake to avoid
Priority is exclusive for exactly as long as its schedule says. A priority playlist with no schedule is always eligible, so it takes the screen permanently and everything else silently stops.
If a screen has stopped showing everything but one playlist, that is the first thing to check.