Blurbs are those snappy testimonials that line the front and back of published books, enticing readers to buy and read. Blurbs mean a lot to me as a reader--often I'll go for a new book because an author I respect has endorsed it.
Agents love when a writer approaches them with a few good blurbs in hand. It's normal for blurbs to wait until your book gets closer to publishing, but it's also good to begin your list of blurb-worthy authors even as you approach final revision.