A blurb is a short piece of writing that describes or summarizes something longer and more complicated. You might browse in the library, reading blurbs on the backs of books to help you choose what to check out.
One type of blurb is written to sell or promote something. A movie's official website might include a blurb describing its plot as well as a short trailer and a cast list, and well-known authors often contribute blurbs full of praise to help other writers market their own books. Any brief written summary can also be called a blurb: in fact, right now you're reading a blurb about the word blurb.