We can measure a circle in two ways, either by its degree or by its arc length. When we walk along a circle of radius 1, we can say we walk 2pi, or we walk 360 degrees.
So Usually, frequency means how many cycles we walk in one second, the unit cycle/s is Hz, but for example, if we say that the frequency is 6280 radians/s, what does it mean?
Recall that one cycle means 2pi, so "how many cycles" means we need to divide 6280 by 2pi to see how many cycles it has in one second, so we convert the unit into Hz.
In all, 6280 radian/s = 6280/(2pi) cycle/s = 6280/(2pi) Hz