Tensors. Years ago, I would read the Wikipedia page about them every so often, and completely fail to understand what they were.

Then one day I was modeling something in a spreadsheet, and I thought to myself "you know, what I really need here, is 3rd axis to this spreadsheet". And for a few minutes, I thought I had invented a new form of spreadsheet/structure, and was considering trying to build a 3d spreadsheet program.

But, of course, suddenly everything I'd read about tensors and not understood before snapped into place, and I realized that this is exactly what they are (to be precise, this is a special case of a tensor, the contemplation of which caused me to understand what the general case is, and why e.g. a scalar is a rank 0 tensor, a vector is rank 1, a matrix is rank 2 my 3d spreadsheet idea is rank 3, etc).