Parked car heater

[sv1cec]

Hey guys, I have a question for you.

There is that silly box inside my engine compartment, at the driver's side. From what I've read, it is supposed to be a parked car heater.

Any idea how it works? I've never seen or noticed it working.

i mean, I know that in Greece, temps very seldom go under freezing point, so I am not crazy about it, and how it works, but I want to know, not nice having something in the car and not knowing how it works.