Red supergiant stars are massive, evolved stars that have exhausted the hydrogen fuel in their cores, expanded to enormous sizes, and cooled so their surfaces glow orange-red.
According to NASA, stars in this class are nearing the end of their lives and are among the largest stars known because they puff up and expand outward as they age. A familiar example is Betelgeuse in Orion: NASA describes it as roughly 700 times the size of the Sun and about 15 times more massive, and notes that such stars will eventually explode as supernovae.