All audio signals need a path to get from one place to the next. And when we're talking about the path they take in an audio circuit, we use the term buss, or bus. This is the physical path that we can use for any of our signals.
And even though in strict electronics terms “buss” means something a little different, in pro audio we use it most often when we're talking about the outputs of a console. So, an aux send could also be called an aux buss. And if an analog console has 4 aux sends, it means it has 4 busses dedicated for the aux sends.
Analog consoles need dedicated outputs like this for each buss because the circuitry is hardwired. But the physical outputs on digital consoles work a little differently. Usually, digital consoles have a set number of outputs, and each output is one buss.
However, these aren't permanently assigned to anything like they are on analog consoles. We can assign any of the outputs, like subgroups, main mix, or Aux Sends, to any of the output busses. This gives you a ton of flexibility with routing.
Additionally, with any output buss we can insert audio devices right in the signal path. Mostly this is used for EQs and compressors. It’s a great way to have overall control of the entire mix on any given buss.