Galaxy reionization is a key event in the history of the universe, marking the transition from a neutral to an ionized state. It refers to the process by which the first galaxies formed and emitted ultraviolet radiation that ionized the neutral hydrogen gas that permeated the early universe. This occurred around 13 billion years ago, shortly after the Big Bang. The study of galaxy reionization involves observing the properties of early galaxies, the intergalactic medium, and the evolution of the ionization state of the universe over cosmic time. Researchers use a variety of observational techniques, such as studying the spectra of distant galaxies, to investigate this important period in cosmic history. Understanding galaxy reionization is crucial for unraveling the evolutionary history of the universe and the formation of structures at different scales.