Netori is the action of cuckolding someone – turning someone into a cuckold.
In this situation, there are three people. One solo, and two that are in a romantic relationship.
The solo person then seduces one of the people in the relationship into a sexual or romantic relationship with them, leaving the other person cheated on, hence being a cuckold.
The solo person is doing Netori on the person that is cheated on.
Other terms in connection with Netori are Netorare, which describes the cuckold, either a willing or unwilling one, and Netorase, which is a willing cuckold, someone who actually wants to be cheated on.
Netori is a common category on adult entertainment sites providing exclusively hentai videos, but can also be found in smaller numbers on usual adult entertainment sites.
“Fakku!” is an online page that provides hentai fans with tags based on specific topics they are interested in, and Netori is one of them. They have approximately 50 hentai tags featuring Netori.