For me it was limit theory. Limits are really a very intuitive subject - almost common sense - but the definition we had to memorize was so complex and dense with unfamiliar math symbols that in highschool it took me a while to figure out what it was trying to say. Eventually I had that "ah hah" moment when I thought "is that all it is? That's not so hard." It's a good example of how the study of math is so much about vocabulary and understanding symbols, and once you crack the code it's not as hard as you might first think.