It's when 20 of your mates and you, stock up on portable radios and use public transport.
Stereo bus...

OK, OK, seriously. A bus...

It's what you call your summed mix. If you're using a mixer (analog or digital) it's the main mix, or any of the monitor mixes. If you're using a recording program or something like that then its any send or aux you create to put effects on.

OMG thats hilarious!

Sometimes it is a group channel. Example you have a 6 mic set up on the drums, each mic has a different volume setting and each are panned differently. You then route this to one stereo channel where you can bring the entire mix up or down without having to move six sliders. A mono group would not allow the pan effects thru.

This is also done when you want to put a global effect on the entire group for example in this case you could use one insert effect like reverb across the entire drum sound. This can save a good deal of CPU as opposed to six channels of verb.