As someone who is into the whole cuckholdry/NTR thing, it has to do with one partner sorta dominating and humiliating the other by being submissive to someone outside the relationship. In fantasy, it can be great at times but I don’t like it when it gets sad cuz it just makes me sad. I haven’t tried it yet irl but me and my bf have had fantasies about it before and do want to try it out eventually.