The Motion Picture Association film rating system is used in the United States and its territories to rate a motion picture's suitability for certain audiences based on its content. The MPA rating system is a voluntary scheme that is not enforced by law; films can be exhibited without a rating, although most theaters refuse to exhibit non-rated or NC-17-rated films. Film ratings often have accompanying brief descriptions of the specifics behind the film's content and why it received a certain rating. They are displayed in trailers, posters, and on the backside of home video releases. Film rating content descriptors are exclusively used for films rated from PG to NC-17; they are not used for G-rated films because the content in them is suitable for all audiences even if containing mild objectionable content. PG stands for Parental Guidance Suggested, and is accompanied by the following text: "Some material may not be suitable for children".

Source: Wikipedia