It refers to a cold fermentation period which may last anywhere from 12 hours to about a week. Instead of making your dough and leaving it out to proof for an hour or two before using it, you stick the dough in the fridge and forget about it for a while. The cold retards the fermentation process which helps to develop more complex flavors in the dough, without developing unpleasant off flavors that you would get if you did the same thing at room temperature. Personally, for New York style pizza I like a dough with at least 48 hours of cold fermentation, and preferably about 5 days. Not only is the flavor and texture better, I find it's easier to work with because the gluten network has more time to come together.