Why and how can jenkins run the same stage in more than 2 agents?

I have witnessed this quite a few times, jenkins runs a same stage on more than one agent. I wonder how could that be possible?

Is it multithreading? but that happens on single machine

is it distributed processing? do we need that for a simple job of 'mvn clean test' ? even if we do, how it will work out? run half test machine A and half on B.

Thanks in advance.