When audio went from analogue to digital, it became possible to label or encode audio files with more information than could be contained in just the filename. That identifying information is called "metadata".
It is used to name, describe, catalog and indicate ownership or copyright for a digital audio file, and its presence makes it much easier to locate a specific audio file within a group - by using a search engine that reads the metadata. As different digital audio formats were developed, it was agreed that a standardized and specific location would be set aside within the digital files where this information could be stored.
As a result, almost all digital audio formats, including mp3, broadcast wav and AIFF files, have similar standardized locations that can be populated with metadata. This "information about information" has become one of the great advantages of working with digital audio files - since the catalog and descriptive information that makes up the metadata is built right into the audio file itself, ready for easy access and use. You no longer need to consult a paper catalog or product packaging to find out more about a particular file.
There is NO metadata on an Audio CD.
When you load an Audio CD into your computer and use iTunes or WinAmp, it may seem that details about the CD "magically" appear on your screen. Actually, the CD producer has supplied the Album Name, Song Titles and Composer information to a huge online database called Gracenotes (or FreeDB). As long as you have Internet access, computer audio players will match the "fingerprint" of the CD in your local drive to the online database and display the CD details on your screen.
If you rip an Audio CD with iTunes or WinAmp or any number of other freeware or shareware programs, the metadata that ends up in your digital audio file comes from either Gracenotes or FreeDB. There are also many freeware and shareware programs that will allow you to edit the metadata in mp3 files manually.