What even is armor proficiency?

[Tanarii]

There's a direct link between the in world, the abstraction, and roleplaying. If your character is not trained* (in world) you take a penalty to certain things that are harder to do while wearing the armor (in world represented by abstraction). If you don't want your character to take that penalty, you decide they don't wear the armor (roleplaying).


*or possibly divine granted the knowledge for clerics.