Ask HN: What's the science behind audio echo on conference calls?

We've all been on Zoom (or conference call app of your choice) where, due to people being in close proximity, or other reasons, there is harsh audio echo/feedback.

What is the science behind this? Why does that occur? We all understand what to do when such echo occurs ("can someone please mute?" "can someone turn off their volume?"), but what actually drives this phenomenon?