For me it’s time signatures in music. I grew up playing instruments, played in several orchestras, etc. But no matter how many times someone tries to explain time signatures to me, it makes no sense. I don’t understand why people place such importance on it. To me, notes have a pitch and a duration, maybe some dynamics. That’s it. I don’t understand why people try to needlessly complicate it. It doesn’t give you anything.
My wife will hear a song and go “Oh neat, it’s in 5/8.” And I’m like “How tf do you know that, and why does it matter?”
Math and numbers in general. I have hyperfantasia so the voice in my head is completely self directed and language is otherwise only external, I read whole words I don’t read letters, when I learned to read it was all at once essentially, like flipping a switch. Numbers don’t have anything visual I can attach to them and don’t exist holistically, I have to look at each number not the whole set or problem or algorithm at once so it’s very foreign to how my mind works. Ideally I could just do everything in geometry because I do have something there to translate to my visuals only brain but as is I just really struggle with math and reading numbers.