The term shutter angle is a useful alternative to describing shutter speed. When using shutter angles we are defining a shutter speed relative to frame rate. The term is legacy from the days of physical shutters and film.
In MAVIS, using shutter angles means you can lock your shutter to a ratio of the frame rate. By far the most common shutter angle is 180°.
A 180° shutter will give you shutter speed which is half the frame rate. Lets say we are shooting at 24fps (e.g. 1/24) with 180° shutter. This would mean our shutter speed would be 1/48 - exactly half the frame rate.
If we were to change our frame rate to 30fps (e.g. 1/30) with 180° shutter angle our shutter speed would instead be 1/60.