Bussing is another name for grouping. It let's you group several tracks and assign it to one or two sliders. During mixdown for example, let's say you have the perfect balance between all the drum mic tracks, but the drums overall are a little low. By assigning all the drums to a couple of busses, you can raise or lower all the drum tracks just using two (for stereo) buss channels.
It's just an easy way to group several things that should be treated as one instrument. Many tracks of background vocals would be another good candidate to send to the busses, or horn sections, or strings.
