The letters 'RR' on a boarding pass signify a Random Review, which means a passenger has been selected for extra security screening. This selection can occur for various reasons, including random checks or specific travel patterns, and it does not indicate any serious concern. It's important to note that the meaning of 'RR' can vary by airline, as different carriers may use the code for internal operational notes. Passengers should remain calm during the screening process, keep their travel documents readily available, and cooperate with airport staff to ensure a quick and efficient review. If there are any questions about the codes on a boarding pass, travelers are encouraged to ask airline personnel for clarification.