Q: What is a buss?
In audio we use the words 'bus' and 'buss'. Either spelling is acceptable, however for this answer we will use 'bus' to mean the road vehicle; 'buss' to mean the component of a mixing console that is under discussion.
Think of a road bus at the outer end of its route in the suburbs of a city. Initially it is empty, but as it wends its way to the city's heart, it picks up passengers at every stop. Eventually it gets to its destination and everyone gets off.
A mixing console buss is similar (in an analog console). It is a wire or metal rod that starts at Channel 1 at the left of the console and ends up at a group output or master output on the right. Let's say that we are considering the buss for Group Output 1.
As it traverses the console, it picks up signals from any channels that are routed to that buss, just like the road bus picks up passengers.
When the buss arrives at Group 1, it delivers all of its signals.
Of course, you can only take an analogy so far. The buss in a mixing console doesn't move, and it doesn't return anything back to the channels (there is no reverse direction to its route).
An analog mixing console has one buss per group, plus one buss for each of the master outputs, plus one buss for each auxiliary send that it has.
Digital mixing consoles mimic the busses of analog consoles. Digital audio workstations also employ the same concept.
In conclusion, wherever audio signals are mixed together, in either the analog or digital domains, you will find busses.