I'm reading Quantum Field Theory in a Nutshell by Zee and I'm completely lost on something. In one of the appendices to chapter I.2, an appendix on Gaussian integrals, he introduces Wick contractions. It seems to be coming out of the blue and I don't understand what he's talking about. I'm sure the math all works out, but I don't see what any of it means, and because of that I also don't follow the generalizations he's making (which I need to if I'm going to do an exercise - right now it looks like I'm just going to be skipping it). Worse yet, if I Google Wick contractions, I seem to find something, well, not completely unrelated, but something quite different. Of course my question sort of hopes that someone has read this book, but maybe I can also be satisfied if someone could just explain to me what a Wick contraction is, what it is supposed to mean, in a physical sense (the mathematical Wiki page isn't too clear to me either).