Don't sweat it, neither do 90+% of the people who complain about it on this site, or whoever wrote the tag definition.
The literal translation is something like "to have [one's lover] taken away via sex". It's a very specific type of cheating, where one partner is, initially, raped or coerced into it, but finds themselves enjoying it much more than sex with their partner, and it eventually becomes consensual.* If it's presented from the primary victim's viewpoint, the thrill comes from watching them grapple with the resulting guilt, shame, and denial until ultimately embracing the pleasure; if it's presented from the viewpoint of the secondary victim, the... "thrill" ...comes from vicariously experiencing their feelings of betrayal and despair at witnessing their partner thoroughly satisfied by another. (In the former cases the cheating may never be discovered by the secondary victim; in the latter cases it necessarily is, otherwise there'd be no story.) If it's presented from the viewpoint of the person doing the taking, it's not netorare, but the closely-related fetish called netori. (That would be your "Grolokk sneaks into high society and bangs out some stuffy dude's hot, younger wife behind his back" scenario.) And then there's a third variant called netorase, where the secondary victim is actually in on it, often without the primary victim's knowledge; this is the closest version to the cuckoldry fetish as understood in the west.
(Note that I've used gender-neutral language because so-called "reverse NTR" stories where a girl steals a guy from another girl exist (and for all I know yaoi/yuri NTR is probably a thing too), but the vast majority of the time the primary victim is female and the taker and secondary victim are male.)
*even this isn't entirely accurate though, because there are NTR stories where the secondary victim is just pining after the primary victim rather than actually being in a romantic/sexual relationship with them, or they're family (with no incest going on, though again, the secondary victim usually wants there to be); in these cases the sex is often even entirely consensual from the beginning. It's a really hard fetish to describe in a way that is both accurate and concise.