The phenomenon you are referring to is known as the redshift. When the wavelength of spectral lines from a galaxy or star is observed to be longer (shifted towards the red end of the electromagnetic spectrum) compared to what is expected in the laboratory or from a reference frame, it indicates that the object is moving away from us. This is a consequence of the Doppler effect, which describes how the observed wavelength of a wave changes depending on the relative motion between the source of the wave and the observer.
In the case of light, as an object moves away from us, the wavelengths of the emitted light waves become stretched or lengthened, resulting in a redshift. The amount of redshift is directly related to the object's velocity away from us. This effect can be observed in the spectral lines emitted by the object, such as absorption or emission lines produced by certain elements.
By measuring the redshift of the spectral lines, astronomers can determine the velocity at which a galaxy or star is receding from us. This provides valuable information about the motion and the large-scale structure of the universe. The redshift of distant galaxies played a crucial role in the discovery of the expansion of the universe and the formulation of the Big Bang theory.