"rule" = the original subreddit's only rule: if you visit, you have to post something before you leave. most posts are just titled "rule" as shorthand for "i'm posting this because of the rule"
where does it come from: a subreddit called /r/196, which was a successor to /r/195. i have no idea why the original was named 195. the "just post whatever" rule led to people posting all sorts of memes there. eventually it formed a community and for whatever reason the community is full of horny bisexual femboys and horny trans people who love blahaj
why are the memes so funny: it was the funniest subreddit and i genuinely have no idea why. i think it's partially because the "post anything" rule leads to a wider variety of content - you might see a recycled meme or two, but you also might see some random image someone saved to their hard drive three years ago and forgot about. variety is the spice of life and also the spice of memes