How to read code that hasn't been commented?

I'm very much an amateur, I just write scripts as I need them, but I'm trying to decipher someone's code and I'm having a hard time because it's not commented. I'm not talking about comments like %define class classdef ClassName but comments saying why things are done a certain way (eg fitting data to a specific user-defined function without ever saying why the function was used, defining specific variables as scalars and