Creaming is the process of mixing fat, butter or solid shortening (i.e. Crisco), together with sugar. The dry ingredients are mixed with the fat until it becomes light and fluffy and increased in volume, due to the incorporation of tiny air bubbles. Not only does the mixture expand, but the color becomes a paler yellow (if using butter). These air bubbles, locked into the fat, remain in the final batter and expand as the item is baked, serving as a form of leavening agent.
Overall, creaming is a very important step when making cookies, cakes, and buttercreams (Captain Obvious on that last one, right?), so certainly don't skip this one. There aren't any steadfast time constraints on this one, but usually two sticks of butter with sugar in the bowl of an electric mixer set on medium-high takes about 3-4 minutes. Use your judgment, and pay attention! This is not the time to walk away from your stand mixer and do an exercise video... who would ever do such a stupid thing... :)