For example, generally, if someone speaks American English fluently, that person would be familiar with not only common American phrases but also could relate to it.
Two strangers cross each other's path and one of them in a friendly gesture, playfully says...Boo! The other person if startled would immediately afterwards realize that it was a joke because the person used "boo" and therefore reciprocate the friendly intent of the gesture by a gesture of acknowledgment...perhaps through a smile, laugh, saying "you got me!", saying "shut up! (while smiling), respond with keeping the banter...etc.
On the other hand, if it were someone that was not familiar with "boo" and what it implied (perhaps due to learning it through text book, rather than experience), they might take it as a malicious gesture. Even though the person might be more fluent than the first person in English, they come from a different culture of where that particular language is used, thus they don't have the cultural understanding that comes with many parts of it.
What is that cultural understanding called?
For instance, if it was called ooly. I would say..."He's good in English, but still, be mindeful that there could be misunderstandings due to ooly."