Just a way of combining a bunch of different signals and sending them to a single output.
An example of what a bus might be used for.
Lets say you just recorded a drum kit:
Kick - channel 1
Snare -2
LO -3
RO -4
Tom -5
Floor -6
Ok, so 6 tracks coming into channels 1 - 6 of your mixer.
Different types of busses for different uses. An effects bus (or aux or whatever it may be called) would allow you to effect each one of those channels in stepped amounts, with some type of processor. Typically reverbs, chorus's, delay's etc.
Other busses could be a subgroup. Maybe you want to have easier control over the whole mix. You could assign the entire drum kit to output bus 1, the rhythm section to bus 2, lead vocals to bus 4, backup vocals to bus 5 etc. etc. These subgroups then typically get assigned to the master stereo output fader on your mixer.
So, it gives you easier control for dynamic mixing. No need to use two hands controlling the drums at some point in a song, just use one fader. Can be used for sending different headphone mixes as well. All depending on how elaborate your mixer is.
How you use the busses on a mixer, or software based mixer is really up to what your needs are.
