Google result:
stands for netorare, which translates to "cheating" in Japanese. While the term is similar to cuckold ("husband of an adulterous wife") the couples involved here are not in a marital affair. In the story, the two girls cheat on their boyfriends by doing things with each other.
So if we see males who are NOT the MC with the heroines, it is NTR. Our game doesn't have NTR and never will, but recent episode has fans concerns that it will. We've released a statement explaining everything.