My experience with actual reality is that nobody ever understands what anyone else says. The goal is only to achieve an approximation which is good enough to function. Better approximations, better functioning.
The reason for this is simple enough, but hard for introverted technical people to deal with. Language is imprecise. (As I've said before, the glory of the English language is its wonderful ambiguity.)
On top of that, our experiences are relational and not declarative, and each person has a different context in which to live.
What we do when we speak or write is to lay out symbols which we hope will inspire in the mind of our audience some semblance of the thoughts which are in our own minds. The very best communicators incite very similar thoughts in numbers of people, but even so no two would reconstruct precisely the same sense of the idea.
Besides, the very best communicators typically rehearse and revise their message many times over. (How many times did Martin Luther King give variations of "I have a dream" to various audiences?) Most of us never get beyond making one faulty attempt to share any particular idea with any one other person.
If no person understands what I say, what makes an "understanding" person? I think that is one who tolerates the inevitable failures in communication and continues to function in a relationship with the other person. In other words, the understanding person is the person who recognizes that I'm talking right past her and listening around her and moves on to another attempt to contact me. Such a person seems understanding to me.
In general, communicating well is a good tactic in the actual reality game, and so, in general, is being resiliant in communication -- being "understanding". But I think we are all so poor at both these things that neither can be elevated into a strategy in itself.