Thought I'd chime in with my own explanation. Before, if you wanted to use a soft synth the first track that was created for the synth was a MIDI track. This is the track that tells the synth what to do, what notes to play, how loud, etc. The input for this track would need to be some kind of MIDI controller (a keyboard) hooked into your computer via a MIDI input. This is what you would specify as the soft synth's input. If you didn't/don't have one you can create your own MIDI synth track several ways, the most common way being the piano roll. Now that the synth has an input, it's output would be the soft synth's audio output. But since this is a MIDI track you can't hear it. The MIDI track is all operational data. It is not hooked up to your computer's soundcard. It needs to go into an audio track since,of course this is set up for audio and you had to hear the synth's audio output throught that. So, you would then need to create an audio track whose input would be, can you guess?. The soft synth. You would then make the output of that track your sound card so you can now hear it. This track is also where the synth's actual audio is recorded so you can mix it down. the instrument track jusy combines these two track into one to make things simpler. The two tracks actually exist but are hidden.