A set of data that describes and gives information about other data.
For our interests, metadata is data about music, specifically about the music we own and play. Album Title, Artist, Album Artist, Composer, Track Title, Track Number, Year, Comments, Album Cover Art, and so on. Ideally when we rip or download, all of this data about our music's data comes along with it. But the odds are this is not always the case. Especially if you've ever moved your library or converted from a Mac to a PC or vice versa, and even more likely if you ever ripped albums as WAV files.