Metadata is information that tells you about other information. For example, the table of contents
in a book tells you what is in the book, how it's organized, and where each part begins. The index
in the back of the book also describes what's in the book, but in a different way.
On a higher level, the card catalog (or its online equivalent) at your local public library is another example. It keeps track of information about all the books in the library: who the author is, what the title is, what subjects it addresses, when it was published, how many pages are in it, and what books are related to other books. There's more metadata in the library, too: look at the signs that show you which part of the library the books are in. Information about where different kinds of information are: that's metadata.
On a web page, the only metadata is often just a title. In other cases, there's also a description and a list of keywords to help you find the page in a search engine. On a well-designed site, every page will also have some sort of navigation tool that puts it into context as part of the site.
And on this writeup, the metadata is at the top: it shows that Sylvar wrote it, that this writeup is about a thing, and when it was written. If you have voted on the writeup, it will show you its reputation. If someone has chinged it, then you will see that information as well. All of this is metadata that describes this writeup.
What's the fuss about metadata, then, if it's already been a normal part of life for such a long time? When there's a huge amount of information to keep track of, such as on the Internet, you'd be lost without some sort of metadata. Can you imagine trying to find a book in the library with a keyword search? "I'm looking for all books that have the words 'science', 'religion' and 'evolution' anywhere in the book." Good luck, pal.