itch.io

You seem like a well-meaning kid, so I'm going to not sugarcoat it. IMO it's better to have this stuff presented in a sanitized environment.

R34 means "rule 34." Basically, it's slang for anything that's sexual content of something, most typically some existing IP. Something like adult-oriented content of two characters from some cartoon.
(Protip: "Adult-oriented" is most often read as a clean way to say "sexual content." If you're not doing that, it's better to just explicitly say what you think is not child-friendly (i.e. graphic violence, vulgar language, traumatic themes, etc). This also has the side-benefit of warning users who might be sensitive to specific kinds of content.)

Furry has a lot of definitions, depending on how vulgar someone wants to paint it, but the most broad definition is "anything to do with humanized animal characters." In that sense, it can range from anything like Sonic the Hedgehog or Zootopia (stylized human/animal mixes) to Warrior Cats or Redwall (animals that just act like humans), but typically a lot of people take it to specifically mean a more humanized style found in a lot of internet subcultures (often animals with human bodies), or someone deep into these subcultures.

So combining the two, it's basically someone who makes really crappy adult-oriented art of something like the Alpha and Omega series (link is just a novel cover for reference).

Anyway, people gave you answers for Itch's policy, but if you want to make something in the meantime, it doesn't really take THAT much to do, but you should start with figuring out what specifically you want to do.