What does PG-13 mean?

Last weekend we rented movies and Kaitlyn wanted to stay up and watch mine. Me and John were dicussing what it was rated and Kailtyn says I know what PG-13 means. "It means there is P and a G on it and there are 13 things wrong with it." We ended up calling my mom since she had already seen it to see if it was that bad or not. Kailtyn got to watch it, but we all laugh at her definition.