You better be older than 18!!
It's a genre of adult manga or hentai. It should be defined as CHEATING/Unfaithful/Adultery. It is too often INCORRECTLY defined as "cuckholding". However true readers of the genre know that cuckholding is just a sub-element and a fetish. A fetish where one likes to watch their significant other, having relations (usually sexual) with another person.