If a movie is for everyone, meaning little kids, it is rated G. PG means it's not totally babyish, but young kids could watch it. PG-13 means Parent Guidance is suggested for children under 13. Rated R means little kids shouldn't watch it, and older kids should have adult supervision. Rated R movies have things like violence, language, etc.
G=General, PG=Parental Guidance, PG-13=Parental Guidance strongly cautioned. Some material may be inappropriate for children under 13, R= Restricted, 18-A= People under 18 years of age must be accompanied by an adult, 14-A= people under 14 years of age must be accompanied by an adult, A=Adult