An audio buss is an electrical conduit that accepts incoming signals from multiple sources. Such as controlled by the master/main fader of a console/mixer being fed by signal from several channel strips. In RB and other DAWs we are taking about a virtual/software buss behaving in the same/similar manner as a hardware/wired device. This BUSS basically sums together the incoming signals from multiple channels into what you are hearing; a stereo, left and right or mono output. Because Busses are so handy at summing input signals to something we can use we use them for a bunch of other purposes also. An AUX Buss or FX Buss is intended just for those other purposes. For example; I can put a Reverb PlugIn on an FX buss, then route/send 5 - 6 vocals channels/tracks all to that one FX Buss for the same reverb effect on all 5 - 6 vocal tracks. That FX buss is being sent/routed to my main/master Buss where it is processed to stereo with my other channels/tracks that don't go through the FX buss.
That's about all I have time to explain. But we can do many other things with busses. Keep in mind most of the terminology used for software mixers and DAWs comes from engineering vocabulary of the hardware mixer/console era before software existed. But now, it's virtual.

Does the noise in your head bother me ?