BUS relates to how separate audio channels are routed through a mixing desk, (real or virtual).
Example 1: a 16:2 mixer has 16 channels that can be simply mixed down to the 2 channel stereo main output
Example 2: a 16:4:2 mixer has 16 channels that can be organized into 4 separate sub-groups (busses) before they reach the 2 channel main stereo output. This is very useful in many ways:
A 4 channel bus can be controlled from one fader - much easier!
Sub mixes can be created for separate groups of instruments: bus 1 = drums bus 2 = electric guitars for example; with different EQ, compression, reverb etc settings for each bus.