What are instrumental improvisations?

It goes something like this:

"One track is first improvised on a multi-track recorder and then subsequently the other instruments are improvised on top of the first track...second track...etc...etc... 


Acoustic guitar, a spanish/classic guitar, twelve string guitar, electric guitars, a dodgy grand-piano, fretless bass, double bass, mandolin, lyres, violin, metalophone, harmonicas, Tibetan siningbowls, electronic vibes, vibraphone, drums, percussion & shitty free drum loops, ...an old broken piano & a crap kids-keyboard: