There is a term in audio recording that is a bit confusing (go figure) to a lot of folks who are not immersed in in the audio or electronics world. Oh, who am I kidding? There are scads of those terms. That is part of the why Home Brew Audio exists - to translate the techno-lingo into the common speech:).
But the term I refer to today is "bus", also sometimes spelled "buss" or mixer bus, though that just confuses the issue even more. But I digress (Really? Me? but that never happens;)).
Anyway, the term, as so many are, is a left-over from the analog days when physical mixers were necessary in audio recording (hint: they are no longer necessary in computer recording). A buss is an output channel on a mixer that has collected all the audio from any channel that is sent to it. For example, If you have a 16-channel mixer, there will be a channel strip for each of those 16 channels.
The "master" channel is actually a buss, because it takes the output of all the channels on the mixer and outputs them to your speakers or headphones, etc. All channel strips on mixers are usually sent there by default. But you can also choose other busses to send these channels to. For example, there is usually an auxiliary (often abbreviated as just "aux") buss knob on each channel strip which lets you send the audio on that channel to the (you guessed it) the auxiliary buss on the mixer. Then the aux buss will output all channels sent to it, and only those channels.
Regardless of how many busses are present on a mixer, there will always be a master-level control somewhere on the board. For example, there will be a "master" strip for the master buss, and there will also be a master Aux section (often not a strip, but a section at the top of the mixing board) with an aux output and input and level control.
There are other types of busses and even other names for the ones I described. For example, sometimes the aux buss is called the effects buss. But at the end of the day, the only thing you need to know is that a buss combines signals from several other places on a mixing board.
We use the same concept on non-physical computer mixers nowadays, which are almost always designed to mirror the way a physical mixer works, though we have a lot more flexibility to create our own virtual busses to create groups, submixes, and any combination of inputs and outputs.
You caught me! I just casually used a term like everyone in the world knew what it meant. Mea culpa! I promise to come up with a suitable punishment for myself. But first - the submix thing.
OK, let's say you have recorded a song with a guitar, a bass, a lead vocal, and 6 harmony tracks (I like to have each harmony part sung twice; so 3 harmony parts will need 6 tracks. I know! Awesome huh?). Then let's say that I want reverb on all 6 of those vocal harmony tracks. I could go to each track separately, and set up a harmony effect on each one. Well THAT'S tedious! Doing the same thing 6 times.
Wouldn't it be cool if I could just set up the harmony effect once, and have all the harmony tracks share it? Hint: the answer is "yes!" And that's exactly what you can do with a buss!
Just create a buss track, set up a harmony plugin effect on it, and then route the harmony tracks through that buss, and they all share like good little harmonies. And...and (this is the best part) you can control how loud all 6 harmonies are with one volume control!
The one on the buss track where they are sharing a reverb (note to the techies - you would want to disable the direct sends from each harmony track to the Master buss first).
So now, if you're listening to the test mix with your wife, and she says (as she always does) "the harmonies are too loud in the mix," you can nudge them down with one slider/knob instead of trying to adjust the volume on all 6 tracks? One track to rule them all! Now isn't that useful? It is.
Hopefully this information will allow you to put another seemingly baffling audio term into your vocabulary.