Jack is a sort of software mixer-board. With it you can route the output of any jack enabled program to the input(s) of any other jack enabled program. This means you can have 3 audio programs (zynaddsub, hydrogen and qsynth, for instance) each routed to a different input in Ardour (a hard disk recorder), the same input, or one going to an input and the rest outputting directly to the speakers.
It sounds fairly complicated, but you can do a lot with it.
Post the output of
That will tell you how many cards you have, and how to route to it in Audacity.
:-$ cat /proc/asound/cards