Rube is an insulting word for a person considered uneducated or uncultured. Your average country bumpkin is also a rube.
Calling someone a rube is another way of saying, "You sound like an idiot and you don't know what you're talking about." This word implies a lack of sophistication, manners, education, and culture. Rubes are usually from rural areas, and they’re also known as bumpkins, hayseeds, hicks, yahoos, yokels, and hillbillies. This is an insulting word, so use it cautiously — though it's probably fine to use it jokingly with your friends.