The fourier transform. Encountered it first in my undergrad engineering degree, it was presented as dry mathematics, with no real explanation, just threw complex exponentials at us, and pages of derivations. Years later I actually use it in my job, and through that and other material can see its beauty, and how its actually not that complicated. Some great resources like this helped a lot:

[0] https://betterexplained.com/articles/colorized-math-equation...