R-18 literally means Rated 18, as in "for adults 18 and up". This encompasses everything from gory and bloody scenes of death, injury, and killing to pornographic scenes of nudity and sex.
Some of you have had a little trouble with this term, so I put the definition of it first. This term is seen a lot in readmes for many models. Even so, many have gotten confused by this term and end up doing things with the model that are actually not allowed.
For example, with Hetalia models, almost all of them forbid violence, blood, and gore as well as anything related to or depicting sex, but all of that is shoved under "R-18" and you don't know what that means. There are some of you who do know what that term means and continue to do this, but that is another matter entirely.
Moving away from Hetalia, there is the "hentai" in MMD. Yes, it is true that many of the things used to edit models come from a hentai game, but that's not the point. Putting aside the models made specifically for "R-18" audiences, most of the models forbid hentai. Sometimes, it is specifically stated by the modeler that this is not allowed.
Sometimes, the modelers put beside the term "R-18" that yaoi/yuri is also not allowed. First, let me make sure we are clear on the definitions of these terms:
YAOI is NOT kissing, hugging, and touching, that is called shounen-ai. It is literally unadulterated boy on boy sex.
YURI is the same thing. It is girl on girl sex.
That, ladies and gentlemen, is called PORN.
And that is also tucked neatly under the umbrella of "R-18".
Violence is also included. It's pretty self-explanatory really. Pretty much anything that is gory, bloody, and just plain revolting is usually forbidden. What modeler wants to see their creation covered in blood? Especially in other cultures, such images are a huge taboo.
Basically, R-18 is anything that would warrant an "R" or above rating if it were in a movie.
Now that you know what it means, you should at least endeavor to respect the wishes of the modelers. Do read the readmes, and if you cannot, translate them. Even google translate can help.
If you have doubts about whether or not what you're doing is right, then don't do it, or try to find a way around it. Everything will be better that way.