A "perfect rolling sphere" is metaphorically an 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. Nonetheless, in a lot of cases, a perfect rolling sphere is a pretty good approximation to a roughly spherical wooden ball moving down an iron trough inside an atmosphere. More importantly, you're probably not going to be able to analyze the 'realistic' case until you understand the perfect rolling sphere.
[summary(Brief): A "perfect rolling sphere" is metaphorically an object that is too simple or idealized to exist in real life. (But which might be illuminating to consider.)]