Technically it's subjective, but for every weapon out there there are a few perks in particular that are 'best' or at least better than other alternatives. Getting a 'God roll' occurs when you get exactly the perks you want/need on a weapon. Basically it's just a way of saying you got a weapon to roll with all ideal perks, which can be pretty rare considering how many lackluster perks there are out there, and how certain combinations of weapon stats and perks compliment each other better than others.