Not running from left to right in a given (& often fixed) tempo, as in DAWs, sequencers…
Not like in a typical classical or popular music, going linearly from intro to chorus to break to…
Imagine you have several “blocks” of ready made music, and several layers of those; or you could see that as a “patch”. The composer must be able to recall these blocks at any given time, and interact with them (that’s the live improv part), and the layers (of blocks/patches) must be able to be run independently of each other (e.g. block 1 on top of block 2, then block 1 again on top of blocks 4 & 7).
It’s a bit similar to what can be done with a mixing console loaded with a whole bunch of tracks (musical parts, or combo’s), where the engineer can fade in/out parts, mute & solo them, etc… but he can pick any place in time of any track.
Ghost Tapes were Mort’s method of putting control voltages on a multitrack tape in his studio, and then playing them back again (through envelope followers) to control his synths et all in a live setting. He could change the playback speed, track “volume”, etc on the fly… So he could always perform something very similar to what he previously did in the studio, but varying for each performance.
Nowadays he uses Live to invoke both CV’s and audio.