Reionization refers to the process by which the first stars and galaxies in the universe began to produce ionizing radiation, which ionized the neutral hydrogen gas that filled the early universe. This process occurred approximately 13 billion years ago, shortly after the Big Bang, and marks a crucial period in the evolution of the universe. Studying reionization allows astronomers to better understand the formation and evolution of galaxies, as well as the overall structure of the universe. Research in this area involves observations of the most distant galaxies and quasars, as well as theoretical models of the reionization process.