Brother.... What...?
Have you only been on the internet since yesterday? Oh boy.
Well, to explain it. A lot of porn is made nowadays. A lot of it is based on fictional or real characters from other media. So for example cartoons. A lot of characters from cartoons have been used in pornographic drawings.
So to the "rule" part. At some point some users of the website 4chan started making up a list of "rules" of the internet. They are mostly just inside jokes, for example rule 29 is "On the internet men are men, women are also men, and kids are undercover FBI agents."
One of those rules is rule 34, which is "If it exists, there is porn of it." followed by rule 35 "If there is no porn of it, porn will be made of it."
Basically it makes fun of the fact that almost every character from TV shows, movies, games, etc. end up having porn made of them. So the term "r34 content" describes porn made of characters from games in this case.