Creaming in Baking.
Creaming is one of those weird terms you see in baking recipes. It does have a specific meaning though, and doing it correctly can really improve your baked goods.
Creaming means mixing the butter (or the main fat of the recipe) and sugar (white or brown) until "light and fluffy". This process puts a bunch of tiny bubbles into the proto-cookies, which help with leavening. You should see the volume of the mixture increase by about a third, and has lightened in color, due to all that air. You shouldn't be able to see any sugar granules, but you should be able to feel them if you rub the creamed mixture between thumb and forefinger. Creaming will take somewhere around 3 minutes with a mixer (I like to use a paddle with a scraper or edge beater in my stand mixer), and just about forever if you're mixing by hand.