"Sublime" is a fancy word that people use to describe something that makes them feel really amazed and impressed. It's like when you see something so beautiful, or so big, or so powerful, that you can't help but feel almost overwhelmed by it. Think of a really huge waterfall, or a stunning sunset, or a really moving piece of music - these things are all "sublime".
In literature, writers sometimes use this word to describe a certain feeling they want to evoke in their readers. Maybe they want you to feel awe and wonder, or maybe they want you to feel scared or overwhelmed. They might use big, impressive words and descriptions to help create this feeling, or they might describe something in a way that makes it seem really important or powerful.
So, when you hear people talking about literature being "sublime", they're really just saying that it's really impressive and makes them feel amazed or moved. And that's a pretty awesome thing!