When linguists — professional scholars of the language — talk about “rules,” they mean the principles that inform the way the majority of people actually speak. These rules are acquired by native speakers more or less unconsciously, and every native speaker knows them, even if they can’t express them. In fact every speaker follows remarkably sophisticated rules without even being aware of it. I like to give my students two examples of rules they know, but don’t know they know. The first is to say, “Both my and mine mean ‘belonging to me.’ So what’s the difference between them?” Most can’t answer, or can answer only after several minutes of thinking about examples — and yet none of them ever uses my or mine incorrectly. (Bonus points to any reader who can explain the actual rule in the comments.) The second is to say, “The ball is red; the ball is big. What is it?” They always answer “a big red ball” — no one has ever said “a red big ball,” even though I gave them the adjectives in that order. No one ever taught my students those rules, and none of my students had given the subjects a moment’s thought in their entire lives. But they all knew the rules perfectly.
These are examples of what linguists consider the real rules.