Wikipedia is wonderful at giving you the handwavey picture, and then you can even skim some more advanced papers with handwavey ideas (e.g. there's a rigorous t

Wikipedia is wonderful at giving you the handwavey picture, and then you can even skim some more advanced papers with handwavey ideas (e.g. there's a rigorous theory of PDEs in graphs relating the Khirchoff/incidence matrix to the de Rham operator, and then developing a theory of how graph Laplacians are really the Laplacians from calculus). But I kind of have a BDSM relationship with mathematics. I'm not happy until it makes me suffer.