What is the R in chess?

Excuse me, but could you clarify what you mean by "the R in chess"? Chess, as a game, typically involves a standard set of pieces and rules, and as far as I'm aware, there's no specific symbol or letter "R" that holds a particular significance in the game's mechanics or terminology. Perhaps you're referring to a specific opening move, strategy, or piece's name that includes the letter "R," but without further context, it's difficult to provide a precise answer. Could you elaborate on your question?