Ask eight blind men what an elephant is. NN is like that. It is no more and no less than a slogan, one designed to evoke a mythical era in which there was an Internet in which all traffic was good, capacity was always ample, and all packets could be treated equally. In other words, NN is the idea that the Internet should be like the telephone network, where all calls are handled the same, except that on the Internet you might have a computer making 10,000 phone calls at once to the same destination. And it's the idea that the government should regulate it to ensure that ISPs all behave that way, even though the actual underlying telecommunications media, which the law says are what's supposed to be regulated, are not. It's the idea that Internet is better left as a regulated monopoly than as a competitive, flexible service.
IMHO, both "sides" of the debate are wrong. The bottom layer (telecom) should be separated from the upper layers (Internet), not regulated or unregulated as a vertically-integrated service, even though it's the ISPs who would in such a case resell the telecom they bought wholesale. It's like flour and bread, where two bread bakeries control 100% of the flour supply in most places, and the government argues over whether Arnold White or Wonder White is the one true bread, while other bakers are shut out.