The "main sequence" is a stage in a star's life cycle where it spends most of its existence. During this phase, stars, like our Sun, fuse hydrogen into helium in their cores, producing energy that makes them shine. The main sequence is where stars are most stable, and they can remain in this phase for billions of years, depending on their mass. Stars on the main sequence vary in size and brightness. Massive stars burn through their fuel quickly and have shorter lifespans, while smaller stars, like red dwarfs, can last for trillions of years. Understanding the main sequence helps astronomers learn about the life cycles of stars and the evolution of galaxies, including our own Milky Way.