Okay kiddo, a blurb is a short summary that tells you about a book or movie's story in just a few sentences. It's like a little preview to help you decide if you want to read or watch it.
Imagine you're in a toy store and you see a big toy box with a picture of a cool toy on it. The blurb on the box tells you what the toy can do, how it works, and what fun you can have with it. Similarly, when you're in a library or bookstore, the blurb on the back cover of a book or the movie poster tells you what the story is about, who the characters are, and what adventure awaits you.
So, if you're ever wondering what to read or watch, just look for the blurb and it will help you decide what you want to do next!