The phenomenon that explains the change in frequency of a wave in relation to an observer moving towards or away from the source is known as the Doppler effect. This effect occurs when the source of the wave, such as sound or light, is in motion relative to an observer. If the source moves closer, the observed frequency increases, resulting in a higher pitch for sound waves or a shift towards the blue end for light. Conversely, if the source moves away, the frequency decreases, leading to a lower pitch or a redshift in light.