DM Review provides a history of metadata and the future of metadata in one article, what more could you want. It is a good overview of the past tools and goals of metadata. Central to providing a solid information application is understanding the data and information we are working with. This understanding allows us to tie is to other information to give that data better depth and this is best done with metadata. Metadata help explains the data and allows for data to be tied (or joined) to other data.
An example would be putting together a quick view of all products sold in an ice cream truck. The driver has a small computer that captures all the sales he has makes throughout the day. At certain intervals he synchs his sales to a central computer so that he can easily stop back at the warehouse and stock up on items he is low on. The driver has sold 7 of ab4532 and 16 of tr78t in the past hour. He has had requests for pp887 too. This information is useful for the computers and the data has meaning, but it needs some metadata to know that he will need 7 of Abners Glow-cicles, 16 Tricky Rootbeer pops, a carton of covered paper cups. This metadata will help the person that will prepare these items for the driver's arrival at the warehouse. The "pp" indicates that a paper product is needed and the number following is its actual product code. The raw data now has meaning with tanks to the metadata.