Perhaps you recall an instance in which a police car or emergency vehicle was traveling towards you on the highway. That was the Doppler effect - an apparent shift in frequency for a sound wave produced by a moving source. The Doppler effect is of intense interest to astronomers who use the information about the shift in frequency of electromagnetic waves produced by moving stars in our galaxy and beyond in order to derive information about those stars and galaxies. The belief that the universe is expanding is based in part upon observations of electromagnetic waves emitted by stars in distant galaxies. Furthermore, specific information about stars within galaxies can be determined by application of the Doppler effect.
Galaxies are clusters of stars which typically rotate about some center of mass point.