What does R stand for in Adopt Me?

Excuse me, could you clarify what the abbreviation "R" stands for in the context of "Adopt Me"? Is it referring to a specific feature, a rule, or perhaps a term used within the game's community? I'm quite intrigued and would love to gain a better understanding of its significance. It would be great if you could provide a concise explanation that highlights its purpose and relevance to the game.