Rules of the Internet refers to a somewhat unwritten, often changing set of rules assumed to be true or necessary.
The Meaning Of Rule 39 Is:
CAPS LOCK IS CRUISE CONTROL OF COOL.
Man1: Hey, whats up?
Man2: NOTHING MUCH, JUST CHILLIN.
Man1: What's up with all the CAPITAL LETTERS?
Man2: RULE 39, DON'T YOU KNOW?