Communication is based on Shared Knowledge

The other day I was speaking with a friend and I eventually used the expression “you've taken the red pill” and he did not understand me. I was assuming that, as long as we speak the same language, we are about the same age and he has seen the movie The Matrix he would understand.

When you ASSUME, you make an ASS of you and ME.
  - Jerry Belson, scriptwriter

He did not remember the part of the movie when Morpheus offers Neo two options: taking a blue pill to stay in a fabricated reality, taking a red pill to escape from that reality into the true one, even though harder and more difficult.

There are phrases in every language that you won't understand, even if you know all the words (are you pulling my leg?). There are shared metaphors (skeleton in the closet, elephant in the room). We also use references to movies, books, TV, … whatever. And the expression will have more content than just the words (go ahead, make my day). Some of them may be understood as part of the language, but some of them are not.

Sometimes you are part of a conversation in a group and they'll start to refer about things they've lived together, and you'll be out of the game.

Sometimes there are terms that are not used with their proper meaning (methodologies, cloud or agility). You may spend an hour discussing with someone and both were meaning different things.

It is not about the language or about the culture; it is just that you don't share the same knowledge. Or the same understanding.

So, if you are in doubt, check for understanding.