If you have a drive that is "full", some tasks may become impossible and some apps may not run, as they would require a little "free space" on the drive (and there is now none left).
If the drive is TOO full, the Mac may not even boot.
But you still seem able to boot, right?
You need to start THROWING STUFF AWAY, or at least moving stuff to an external drive to "free up" space on the internal drive.
You should have 15-20gb of free space at all times.
You may need to toss A LOT of stuff out -- perhaps old videos, etc. ???
Or perhaps time machine "snapshots", if you use tm ??