A "perfect rolling sphere" is metaphorically any sort of object that is too simple or idealized to exist in real life. It derives from an old joke about a physicist who tries to bet on horse races starting from the assumption that each horse is a perfect rolling sphere. It remains true that in many cases you can't hope to analyze an approximately spherical wooden ball rolling with friction and air resistance, if you can't even analyze an ideal rolling sphere; or that an ideal rolling sphere is actually, in real life, a reasonably good approximation of a wooden ball rolling down a not-perfectly-smooth surface in an atmosphere.