Rule 34 was something that became big on 4chan back in the day: it basically posits that if it exists, in any form, porn of it will also exist. The basic idea is that people submit various forms of adult content that are quite strange or interesting and featuring ideas or entities that aren't typically portrayed in a sexual fashion. That sounds confusing, so I guess a few examples will help.
The most upvoted content on /r/rule34 includes a Wonder Woman cosplay, Elsa in a J-Law pose and some photos of a guy inserting Gamecube controllers into the console in a very suggestive fashion. Did I mention that Rule 34 isn't the typical place to come across good quality porn?
Anyway, with just just over 125,000 readers, submissions flood in about once every hour. If you have your own rule 34 material to share, you might want to consider Reddit as the go-to place to do exactly that.