In the domain of random access digital audio virtual tracks refer to tracks that are present in a particular session, but are not able to be heard because other tracks are currently using up the available playback resources of the hardware involved. All random access digital audio hardware has limitations to the maximum number of simultaneous tracks that can be heard on playback. In order to hear any virtual tracks they must either be selected as a playable track (which will likely involve deselecting another track that was playing) or some sort of submix operation will have to be done that will allow all tracks to be heard.
In the domain of MIDI production virtual tracks refer to tracks that exist only in MIDI (as opposed to on a tape machine). A technique that became very popular in the 1980’s was to never record MIDI instruments to tape. Instead a MIDI sequencer is synchronized to the tape machines and the MIDI tracks are always “flown in” and mixed in real time as if they are live instruments playing along with the tape. The sequencer stays locked to the tape machine throughout all tracking, overdubbing, and through mixdown. This allows artists to have many more effective or virtual tracks available than what their tape machine would otherwise allow.