A computer music library consists of a set of music files, normally stored in some sort of file structure. Each music file contains the audio data, which is the music we listen to. However, music files can also contain descriptive data about the music, and this is what is called a tag.
Tags consist of a number of fields. There's a wealth of different information that can be stored.
Music players, when they read the music files, not only read the audio data so they can playback the recording contained, but they also read these tags. The tags can then be used to display information about the music - this obviously helps you to search and browse your music library.
If you didn't have tags, you wouldn't be able to find your music very easily. And hence, that's why it's important your tags are present and correct!
Your music can be acquired from multiple sources, and those sources don't always get the tags correct. For example, when ripping CDs, crowd-sourced databases such as FreeDB are used to look up tags, but the data that was originally inserted into the database may not be correct.
There are many different types of error. Some can be simple punctuation or capitalisation errors. Some can be semantic or syntactic errors, which are better dealt with by bliss's other consistency rules. But plain, simple, just-blatantly-wrong data is something that bliss's correctness rule can identify and help you fix.
bliss attempts to fix these problems by matching your music to more authoritative online databases, comparing the tags, and suggesting fixes when the tags do not match.