It’s a big yellow vehicle that carries children to and from school…
But seriously, a “bus” in audio refers to the path between two points. It’s a holdover from analog hardware, where a “bus” is actually a physical wired connection. In fact, the school bus is actually a good analogy; the school bus carries children from one place to another, and the audio bus carries audio from one place to another.
There can be many different busses. You will probably hear the main outputs, the left and right components of a stereo signal, be commonly referred to as the “2-bus” or the “Master Bus”. It is the “bus” that carries the main two channel signal (hence “2-bus”).
Pro Tools, and other DAWs, use busses to route audio internally. Pro Tools uses the term “bus” for any audio path created and used entirely inside the software, and can use the busses to route audio from one track to another, or split the signal from one track to another, or any number of other uses.
s In very simplistic terms, you could refer to an audio bus as “how to get audio from one place to another”.