ID3 is a tagging format for MP3 files. It allows metadata such as the title, artist, album, track number, etc., to be stored in the MP3 file.
Here's some explanation about the Ignore_ID3 and The NonID3FileNameFormat:
If Ignore_ID3 is set to TRUE, MythMusic will try to determine the Genre, Artist, Album, Track Number, and Title from the filename of the mp3 file. The NonID3FileNameFormat variable should be set to the directory/file format where the mp3 files are stored. For instance, I store mine in the above shown Genre/Artist/Album/Track format. MythMusic will then use this information to fill in the proper fields when it populates the musicmetadata table rather than searching for an ID3 tag in the mp3 file.
The files can be laid out in any format, such as:
Genre/Artist/Album/Title Artist/Genre/Album/Title Artist/Album/Title (with Genre left as Unknown)
The track number is optional but can be specified with the title by using the TRACK_TITLE keyword instead of TITLE. If TRACK_TITLE is used, then the filename can have a space, hyphen, or underscore separating the track number from the track title. Keywords are case insensitive, so if you specify GENRE it's the same as Genre in the format field.
The Ignore_ID3 option does not disable the code that determines the track length, just the portion that tries to read ID3 info.
Note that if the structure of the directories is different than that as described in the NonID3FileNameFormat field, newly placed music will simply not show up when you select "Scan for New Music".