Frequency shift refers to the change in frequency of a wave, often observed in sound or electromagnetic waves. This phenomenon can occur due to various factors, such as the relative motion between the source and the observer. For example, when a train approaches, the sound waves compress, resulting in a higher frequency, while the sound waves stretch as it moves away, leading to a lower frequency. In astronomy, frequency shift is crucial for understanding the Doppler effect, which helps scientists determine the movement of celestial objects. By analyzing the frequency shift of light from distant stars or galaxies, researchers can infer whether these objects are moving closer or farther away from Earth.