Meaning of “NTR”

“NTR” is an abstraction for “NeToRare(寝取られ)” which means get cucked.
“Ne(寝)” if from “Neru(寝る)” which means sleep. (In this case, having sex). “Torare(取られ)” is from “Torareru(取られる)” which means to be taken or to be stolen.
This is an internet slang.
In some cases, there are people who get excited by NTR.

Example Conversation

A: NTR genre can be so exciting, right?
B: Seriously? Would you be okay with it happening in real life?
A: No, I wouldn’t like it in real life, but I like that genre.

A: NTRのジャンルって興奮するよな
B: マジで？リアルにおきてもいいの？
A: いや、リアルだと嫌だけど、そういうジャンルは好き