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 Pro Camera, 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.