RP stands for "rating pending" in reference to video games that are as yet unrated. A rating is applied when a game is advertised and before submitting them for review by the Entertainment Software Rating Board.
Ratings are assigned by the ESRB, so people can make informed choices when choosing video games. Game publishers are not allowed to ship games to retailers without a rating assigned by the ESRB. Specially trained raters undertake the rating process as a group, examining specific criteria to grade each game. Factors considered include violence, language, sexuality and drug use. These factors and weighed with the context and message of the game as a whole to define an appropriate rating.