Definition: ID is used as a placeholder in tracklists and setlists for unknown information, such as the track name or artist. When neither the song title nor the artist is known (or not able to be revealed on the tracklist), it is written as ID – ID.
Tracks that are released in this way are called “IDs.” (Not to be confused with government IDs, i.e. identification cards.) ID is technically shorthand for “identification,” though you’ll never see it written out that way.
Sometimes tracklists are updated later on to reveal the IDs, after the person compiling the tracklist has had a chance to find the missing information, or after the song or artist is eventually revealed by the DJ.
Track or artist names can be unidentified for a number of reasons. In sets, mixes, and podcasts, some DJs may choose not to identify a song or artist because the song may not be finished, or doesn’t yet have a title; because the DJ may not have permission from the artist to play the song; because the artist has not yet debuted the song; or another reason. Sometimes DJs play unidentified tracks to “test drive” the track and gauge reaction to it. If the track goes over badly, they can scrap it without any damage done to their brand as a DJ or producer.
Of course, if the person compiling the tracklist is not the DJ, but instead a fan or someone not affiliated with the DJ, you may see IDs in the tracklist just because that person isn’t familiar with the song or artist.
Further Reading: Max Graham has a great explanation of his use of IDs on his Facebook page, laying out other reasons a DJ may choose not to identify a song.