It's a vehicle that your sound gets on to take it places.
Or here's a better explaination I found..
A bus is basically a path in which you can route one or more audio signals to a particular destination. Destinations can include groups, auxiliary sends, stereo mix, foldback or monitor. Commonly busses are used to route channel signals to a master group fader, a multitrack recorder, or the main stereo master fader (or all).