A buss is an additional output from the channels. They are very important in live sound.
Your mixer has eight pre-amps. Each pre-amp feeds a variable amp controlled by the gain slider or knob. That feeds the main (stereo) mix. Then you can control a feed to auxiliary busses through separate controls. You can feed one or more inputs to the aux buss, and there is a separate gain control for each. According to the manual, yours would go to an effects output for external processing.
The aux buss can feed an internal effects channel, an aux output, or, on a bigger mixer, get mixed into the mains through a separate slider. I do mostly live sound. I use the main buss for the house speakers, and I use an aux buss for recording. A grand piano doesn't really need any amplification (and the pianist might get upset if you try), but I mic the piano and only feed it to the recorder on the aux buss. If I were recording a drum set, I would mic each piece of the set and balance all eight or so channels on a buss. Then I could use a single control to mix that buss into the main mix without changing the balance.