In hindsight, a better word would have been a mild form. But think of it like this, if you have a highly open relationship, then you are basically fully dating other people while remaining with your partner. However, not many people are a fan of this kind of openness, so they have a kind of less restrictive monogamy. So it is more where a couple is mostly monogamous but a little bit open to one or the other or both to a low extent having some fun elsewhere.

This could be the couple partaking in a low level of swinging together, or one of the other allowing the other to engage in that type of activity. It could be that they allow each other the freedom to have a little bit of a flirt with others with boundaries, even to go a little bit beyond that, it could even be allowing each other to visit sex workers. It’s surprising how common it is becoming for couples to be willing to allow their partner to visit sex workers, many are even sending their partners there as presents i.e. instead of a conventional massage a different kind of one.

So yeah, basically it’s just where couples loosen the boundaries of monogamy a little bit to allow one of the other or both to have a little bit of fun elsewhere in some way, but without going for the full shebang of an open relationship.

Hope this helps!