What Does "Metadata" mean?
Metadata--literally, meaning "data about data"--(you can't make this stuff up!) is used by search engines, libraries, bookstores, etc., to help people categorize and locate books. Metadata "tags" are like individual keywords--like the things you see on Amazon in the tag list--words that people might use in searching for your book, or a book like yours. The Metadata "description" is a paragraph--think of it like the blurb on the back of your book. What will hook people into reading it?
There's a great article on PBS.org, from 2010, that explains metadata in minute detail, and it's well worth reading; you can find it here: http://www.pbs.org/mediashift/2010/10/a-self-publishers-guide-to-metadata-for-books285.html . I highly recommend reading it, in order that you can understand, and take advantage of, the metadata fields (keywords and description) available to you, not only at Amazon, Nook, etc., but as part of the information that we embed, invisibly, inside your ebook, helping search engines and retailer searches find your novel, biography, poems or other book.
Need help figuring out what keywords to use? Search Amazon for books in your niche or genre that are at the top of the charts, and see what keywords (tags) they are using; compile a list, and then pick the ten best. Don't reinvent the wheel!