Google it so it doesn't turn into a 3 page discussion of what is and isn't NTR. Basically the girl is sleeping with someone other than you.
lol yeah, that topic can derail a whole thread in no time, but yeah, its basically the definitions given above.
ill just add that it has two main variants of the topic
Netorare=others cuck you
Netori=you cuck others
just so our fellow member
@benx1 can have a broader view on the topic (just be warned that some of the stuff you can find on the topic is so heart wrenching that not everyone can stand it, that is why most games here that contain it warn you beforehand, so you dont get scarred for life.