A Red Giant is a type of star that has reached a late stage in its evolution. After exhausting the hydrogen fuel in its core, the star expands and cools, resulting in a larger size and a reddish appearance. This phase occurs after a star has spent most of its life in the Main Sequence stage. Red giants can vary in size and brightness, often becoming thousands of times larger than the Sun. They are typically found in the Hertzsprung-Russell diagram in the upper right section, indicating their cooler temperatures and high luminosity compared to other stars.