In astronomy, there are two terms used to describe an object's motion in space: redshift and blueshift. When an object is moving away from us, it is called redshift. On the other hand, an object experiencing blueshift will be moving closer to us, and its wavelength decreases as frequency increases.
This phenomenon is related to the Doppler effect in physics, which is attributed to an apparent change in the frequency of a sound wave as noted by an observer due to motion. For example, a passing ambulance has a siren that seemingly increases in volume (as well as frequency) as it nears you, and then decreases as the vehicle moves further away. The siren's fluctuating volume is a result of sound waves either compressing or spreading out. The waves are compressed as they move towards the observer, and the observer perceives the wavelength as shorter and the frequency as higher. The same principle applies to light waves.
The terms "redshift" and "blueshift" come from the visible light spectrum. The blue part of the spectrum has shorter wavelengths and higher frequencies, and the red part of the spectrum has longer wavelengths and lower frequencies.