SweepTest: how it works ?

Hello!

Now, I'm working on multiple unit selection task. So I surfed over the internet and found that some people used the SweepTest function to detect it. However I am not sure I understand how it works.. I mean when you cast a ray it is clear for me, but what SweepTest do inside I don't know and I can't figure out the complexity of that function. I would be very appreciated for your answers!

Thanks in advance.