You’re at the theater and before the movie starts, you see the familiar green background with the words "The following preview has been approved for all audiences by the Motion Picture Association of America", meaning that the movie studio met certain restrictions on how much violence, sexual behavior, and offensive language can be shown.
New yellow and red warning messages are coming into play on the Internet, allowing movie studios to produce racier trailers. Yellow messages are for films rated PG and PG-13 where there is some violence and sex in the trailer. Red messages are for films rated R and NC-17 which have even fewer restrictions on content. The trailers containing these warning messages can only play between on websites with a mainly adult audience or between the hours of 9pm and 4am.