Imagine a point rotating on a circular path. Also imagine a normal line drawn from the point and its foot always touching a diameter. Again imagine radius connecting the point from the center. As the point rotates , the foot of the normal line will oscillate on the diameter(simple harmonic motion). And also as the point rotate, the radius connecting the point will be sweeping the angles. The sine of those angles will make a wavy graph.
Hence rotation, oscillations, and waves are similar motions and can be related. Remember rotation of a coil in a generator produces wavy alternating current.