‘Bussing’ simply refers to sending and receiving channels internally in your recording software.
Say you’ve recorded three vocal parts and you want them all going to a reverb. Well, opening a reverb plugin on every track is going to eat up your processing power, so why not send all three tracks to one reverb? Open a stereo reverb (or any other effect) on a stereo aux track, then by using the sends on your software mixer, send each track to be effected on bus 1-2. Now, set the input of the aux track (where you would usually select an input from your audio interface) as bus 1-2. Your vocals, as well as playing on their own tracks will be sent via a bus to the reverb track. You can now control the overall volume of the reverb (in relation to the dry signal from the vocal tracks), and have control over the level and panning of each vocal track going to the reverb. So, you can still have control over where each vocal is placed in the mix and how much reverb it has, even though you’re only using one reverb! Genius.
The music I’m currently working on has lots of crazy, effected guitars. So one of the techniques I am using, is panning the guitar to the left, bus it to a mono aux channel with a weird effect on it (100% wet) and pan that to the right. Now I have a weird echoey noise on the right channel whenever the guitar is played on the left channel. It’s fun!
Another use for busses is something that I do very often to cut down the amount of tracks I’m handling (usually because there are so many channels in use on my mixer that I have to view it on two screens!). Say for instance I have 10 channels of drums in my session, and I’m still at the tracking stage. I don’t want to mix my drums down yet, not even into sub mixes, so I bus them. I send all of my drum channels to a stereo bus (For example, bus 1-2), open a new stereo aux track, and set the input as bus 1-2. I can then hide all of the individual drum tracks, thus removing them from the mixer. Now my 10 channels of drums has been reduced to one handy stereo channel. My mixer (and head) is now clear for other instruments. This also makes it much easier to control custom headphone mixes.
For small/basic sessions there won’t be much use for audio busses, but for large/complex mixes (and when crazy effects are in order) busses are very useful and worth knowing about.