Mathematical proofs. I studied math in college and didn't really understand anything I was doing until well after school. When I read Book of Proof a few years

Mathematical proofs. I studied math in college and didn't really understand anything I was doing until well after school. When I read Book of Proof a few years later it all suddenly clicked for me and made perfect sense. Half of studying math is just surviving. Same thing for my Fourier analysis and signal processing classes. I didn't grok it until a few years after my classes. Don't even get me started on statistics which I still find difficult to really understand "why do I trust this magic formula?". The hard science behind statistics is actually incredibly complicated when you start to dig into it