In this simple concept, hardware is immersed in a nonflammable dielectric liquid that boils on the heat generating electronics. Because the enclosures are not hermetically sealed and they are not under pressure (just like a freezer, the lid is simply closed when access is not needed), it's also called open bath immersion cooling.
If it sounds a little radical, know that the boiling point is very low (ie. 34°, 49°C, depending on fluid) and that passive 2-phase (evaporative) immersion cooling has been around for decades to cool high value electronics in tens of thousands of systems including super computers, industrial applications and radar or microwave relays. The key difference is that all these systems are sealed and under pressure.
Passive 2-phase immersion cooling (also known as open bath immersion cooling) eliminates all these complexities and makes the system very practical.