Longinus was a man who lived a long, long time ago in ancient Greece. He was very interested in literature, specifically stories and poetry. He thought that good literary works had to have something special and exciting to make them powerful, something that made people feel very strongly when they read or heard them.
Longinus believed that this special thing was called "sublimity". Sublimity is like a fancy, fancier way of saying that something is really, really, really good and inspiring. Longinus thought that sublimity was the key to really great literature.
He wrote a book called "On the Sublime" where he explained more about this idea of sublimity. He said that good writing has to create an emotional response in its readers or listeners. Sublimity can make your heart race or your hairs stand on end. It's when you read or hear something so beautiful, so awesome, or so scary that it makes you feel things so strongly that you almost can't handle it.
Longinus's ideas about sublimity were very influential, and many people followed his writing and ideas for hundreds of years. Even today, people study Longinus and his ideas about sublimity to better understand what makes literature great.