Yeah, fandom has a lot of abbreviations, doesn't it. Fanlore.org is a great place to look them up, it's a wiki for fandom. But just in short:

RPS is "real person slash", i.e., Dennis & Igor in a sexual relationship (rather than Roman & Deniz), strongly discouraged by the actors (and most fans who are able to distinguish fiction from reality.) (BTW, what you wrote, RPG, is "role playing games" and is completely different.)

Non-con is short for "non-consensual sexual situation", i.e., rape as well as other non-consensual situations. Close but slightly different is "dub con" which is "dubious consent."

Mpreg is "male pregnancy". Don't ask me about that, it's just squicky.

"Talking heads" in videos are when one of the characters is talking (or at least his mouth is moving as if talking) and is a no-no in quality vids.

h/c is "hurt/comfort" fic, something bad happens to one character and the other comforts.

HEA = "happily ever after"

OOC = "out of character", e.g., Deniz having sappy romantic thoughts out of context or Roman wanting to play football.

AU = "alternate universe", e.g., Deniz and Roman in Camelot.

... I think that covers most of the ones mentioned here? I hope that's helpful; it can be confusing at first, there are lots and lots of them.