The PG rating is meant as a warning to parents to check out the thread/post for themselves before allowing their younger children to read/watch it. PG thread/post may contain elements that parents may find unsuitable for children in the opinion of the ratings board.
PG thread/post may contain some limited profanity; violence may be depicted a bit more graphically; brief nudity may be present but these elements are not deemed intense enough to warrant a stronger rating.
Bloody violence will usually garner a PG-13 or above rating as will use of the F-word.