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Nothing 'exactly' explainable that I'm aware of, a bus in electronics is typically a method for distributing signals, audio would imply the signal is audio. Other than that the question doesn't really make a whole lot of sense.
A generic bus is a collection of connection points for related signals that can be accessed by a number of different devices or circuits.
I'm currently building a system that uses an 8-channel audio bus. Each bus channel can be either an input or an output for 4 preamp channels, two power amp channels, a two channel audio mixer, and assorted audio connectors, as well as supplying inputs to transmitters and other circuits. (in case anyone is wondering, it is part of my "Penny's Computer Book" project)
This is just one example of what a typical audio bus might be.