What does the Doppler effect explain?

The Doppler effect describes how the frequency or wavelength of a wave changes when there is relative motion between the source of the wave and an observer. When the source of the wave moves toward the observer, the waves compress, leading to a higher frequency (or shorter wavelength), which is commonly referred to as a "blue shift." Conversely, when the source moves away from the observer, the waves stretch, resulting in a lower frequency (or longer wavelength), known as a "red shift." This effect is applicable to all types of waves, including sound and electromagnetic waves, making it a fundamental concept in understanding wave behavior in various contexts, from astronomy to everyday phenomena like the sound of a passing siren.