Here's wikipedia. Granted, the mathematicians behind wikipedia seem to be focused on making everything as notationally obscure and hard to understand as possible, but it outlines the concept https://en.wikipedia.org/wiki/Linear_map#Matrices
Well, I have to say that if that is the most approachable and easily googled content in the internet to figure this out, I am not too surprised I had to figure it out on my own...
