What’s something most people don’t understand?
Each other. This is the first thing popping into my mind after reading the question.
I know it’s a silly answer, but there happen to be very many people I’m sure you have come across saying to you, “ I don’t know what you mean. Or I don’t know what you’re talking about. Actually I have heard some people say, you’re making no sense at all. I never understand you.
Yet, you have no trouble talking to anyone else. So, when you ask what’s something most people don’t understand, my answer is each other.