It is basically exactly what it says on the tin - someone who claims to "kin for fun," or in longer words that no one but me has ever used, to "kin" as an action done for the purpose of having fun. That is, after all, where the term came from. (It's also why I'm more than a little taken aback at how many people are misunderstanding it to mean something else.)

Generally what that means is that they're using "kinning" to mean something along the lines of relating to, projecting onto, roleplaying as, vibing with, or otherwise really really liking a character/thing. Probably part of the reason people have trouble defining it as a one-sentence deal is that it covers a lot of things!

What it explicitly means, though, and this is the important part, is that they do not, by their own admission, actually identify as their "kins" - if you actually identify as the thing in question, you're not KFF, full stop, no matter what else is going on (you use "kin" as a verb, you have a "weird" kintype, you don't take yourself super seriously and like to joke around and have fun with it, etc. etc.). The whole argument is about whether or not it's okay to use the word "kin" to mean something other than identifying as nonhuman/fictional in some capacity.

because this