What does PG-13 stand for?

Answer 1

PG stands for Parental Guidance, the 13 means "13 and up"

PG-13 was created as a middle ground between PG and R, for movies that were not considered appropriate for younger children but not bad enough to be restricted to adult viewers.

Answer 2

Parents Guardian-13

Parental guidance needed since the movie is probably not suitable for children under the age of 13 due to theme, language, or violence.