Music theory. I know really basic **** (like how to stay on key), but that's like scraping the top of a huge iceberg

There's so much to it and I don't think I'll ever be able to fully grasp it. I'm slowly learning things just through experimentation and trial and error though.
I'd also like to be able to learn a new language, but learning a whole new set of grammar rules, characters, etc. is all so much. I've seen some things that are supposed to make the process easier/more efficient, but I've yet to look into it.