If you look at a uniform circular motion from the side, i.e. project it onto a straight line, you get a harmonic oscillation. One observes the change over time and the uniform circular motion becomes with the path speed described.
The radius means the maximum deflection, i.e. characterizes the amplitude of the oscillation.
The amount of speed changes over time because a different proportion of the web speed at any given time when the circle falls in the projection direction. When the middle position is passed, the speed is with is maximum and at maximum deflection .