Cosmic reionization is a key epoch in the history of the universe when the first stars and galaxies formed and began to emit intense ultraviolet radiation that ionized the neutral hydrogen gas that filled the early universe. This process transformed the universe from a state of mostly neutral gas to one that was predominantly ionized, leading to the formation of the universe as we know it today. Researchers study cosmic reionization to understand how the first structures in the universe formed and evolved, and to shed light on the processes that drove this important transition in the universe's history.