In the film context what does the movie rating mean?

The arrival of more and more wide appeal adult content led in movies made the international film importers and disturbers of America and the Motion pictures of Association of America, to devise a new rating system to help parents protect their children from mature material. The rating system originally consisted of four ratings and now includes five.

The rating board process is largely subjective and is ever evolving. A policy Review Committee comprising MPAA and NATO officials monitors the Review Board and provides guidelines to follow when rating movies. At this time, the Rating Board rates movies as follows:

G -GENERAL AUDIENCE - All Audience Admitted: this is applied when film has no nudity, sexual matter/ content, drug abuse or strong language. When violence is minimal and the theme of the movie is deemed appropriate for young children. This does not mean that the movie is a children’s movie though.

PG- PARENTAL GUIDANCE SUGGESTED- some content may not be suitable for children-
The rating board applies this rating when the members believe the film contains themes or content that parents may find inappropriate for younger children. The film can contain some profanity, brief nude scenes or violence, but only in mild form. And a PG film should not include any drug usage.

PG-13- PARENTS STRONGLY CAUTIONED- some content in the film may not be suitable for children under 13 years of age:- 
The board had given this rating to denote films in which violence, profanity or sexual content is intense enough that many parents would not want to expose their younger children to the film. But not so intense as to warrant an R rating. Like any featuring of drug use will get at least PG13 rating. A PG 13 movie can include any harsh usage of words or sexually derived words as long as it is only used as expletive, not in a sexual context.

R - RESTRICTED UNDER 17 REQUIRES PARENT OR ADULT GUARDIAN: 
This rating means high level of adult content. Such as harsh profanity, intense violence, explicit sexual content and extensive abusing drug usage in some states, the minimum age to see an R rated movie unaccompanied is 18.

NC-17 NO ONE 17 AND UNDER ADMITTED - Originally called X, this rating is applied to films the board believes most parents will consider inappropriate for children. It indicated only that adult content is more intense than in an R movie, it does not imply any sort of obscenity. The minimum age to  see N 17 is 18 in some states.