It is associated with the cuckolding fetish, which induces jealousy in the watcher, in certain cases resulting in sexual satisfaction.
Most often than not, it is the protagonist’s wife or girlfriend who is cheating, although in some cases it is the female becoming a cuckquean.
The word is Japanes used for cuckolds, although its literal meaning is “taken away by sleeping” which pretty much describes the whole significance of the word and the name of the subgenre. The first entry of it was added to Urban Dictionary in 2012.
Spread and Usage
The most popular show was aired in 2010, called Triangle Blue. Ever since then the Google searches on netorare have been increasing, despite the controversy amongst anime fans.
One can research further into the stories on TvTrope and Encyclopedia Dramatica.