This is the 823rd most frequent Russian word.
правило
Rule.
Here, 'правило' is used in the sense of 'guidelines' or 'principles' that determine how the language operates.
В школе мы изучаем правила русского языка.
At school, we study the rules of the Russian language.
In this context, 'правило' refers to a prescribed guideline or habit as a part of an organized routine.
Обычно я встаю в семь утра по правилу хорошего режима дня.
Usually, I get up at seven in the morning according to the rule of a good daily schedule.
This phrase uses 'правило' in a proverb to express the idea of a general principle that is validated even when exceptions exist.