Creaming butter and sugar, or shortening of any kind and sugar is an important step in baking. First, the friction of beating actually aids in dissolving the sugar, and second proper beating incorporates air into the mix making for a lighter baked good. To properly cream, place room temperature (slightly cool to the touch) butter or shortening in the bowl and start beating until you can see a change in texture that it is getting softer, then gradually add the sugar, don't just dump it all in at once. Once it is all mixed in stop the mixer and feel how grainy the mixture feels. It will depend on your mixer how long it takes, but probably 3-5 minutes. It should actually look "light and fluffly" and when you feel of it it should feel only slightly grainy. I know this is way more than you probably ever wanted to know! But it is an important step in baking and makes for a nicer finished product.