A Bus or Aux Channel is more or less a channel to which you can route the signal from each channel.
Usually you use one of the Sends from one ore more channels and send it to a Bus. The Send part of the channel has a Volume control (Fader or knob) and splits the signal (without changing anything in the channel) from the channel and sends it to a Bus.
Now you have two channels with the same signal. On the Bus you can then change the signal, e.g. by putting a reverb on it. With the volume fader of the Bus you can now control the overall volume of the Bus channel, which in case of reverb will probably be the volume of the reverb from many different channels, because the Sends from the different channels can all go into one Bus.
In this example the Send Volume controls on each channel control the amount of reverb for each channel, because they control how much from each channel goes into the Bus and so into the reverb.