It’s kind of similar to a crush, or to author Dorothy Tennov’s concept of “limerance”, described in her book Love and Limerance: The Experience of Being in Love. In a monogamous context, the point is that when it’s new, a relationship can (enjoyably) consume quite a lot of your energy and attention, and over time you discover whether you’re really In LoveTM — in the beginning, you can’t really tell, because a new relationship is likely to feel that way anyway. (I should say that I never read Tennov’s book, and all I know of her concept of “limerance” is what I picked up from a short radio piece on the book when it first came out in 1987, so I may be misrepresenting the concept.)
I’ve only heard NRE used in a poly context, and what that context adds to the idea of limerance is the fact that a New Relationship can coexist with an established older relationship or relationships. In that situation, NRE can be a scary thing for an existing partner, who sees this new relationship taking up a perhaps inordinate amount of their partner’s time and attention.
(To make this easier to talk about, let’s say that A and B are in an established relationship, and B gets involved with C — so B-C is the new relationship with all the NRE.)
The concept of NRE is useful to A, because it lets A realise that just because B is spending an awful lot of time and energy with C — maybe even more than with A now — that doesn’t necessarily mean that B loves C more than A, and it’s not necessarily a permanent state of affairs. It could just be that B and C are enjoying a burst of NRE.
The concept of NRE is useful to B, because it gives B an opportunity to do a reality check, and realise that as tempting as it is to take a couple days off of work and go rent a cabin in the woods with C for a long weekend of soulful gazes, hey, it’s still important to nurture the established relationship with A and make sure A knows B still loves zir. All that NRE is wonderful and is definitely worth enjoying, but you may need to pay conscious attention to not magnifying the new relationship out of proportion to the place it’s actually going to play in your life.
And the concept of NRE is useful to C, because it lets C understand that just because B wants to take zir to a cabin in the woods for a long weekend of soulful gazes, that doesn’t necessarily mean that C is going to become B’s primary partner, and the relationship may well settle down to a less intense phase at some point. So C can avoid building up unrealistic (or even realistic, but just incorrect) hopes and expectations.
As I write this, it sounds a bit negative, but actually, that’s not quite how I see it. The poly relationships I’ve watched deal with NRE have mostly accepted it as a delightful and enjoyable thing — and if you’re aware that it can sometimes cloud your perceptions and take care to keep at least a tenuous grasp on reality, you can have a lot of fun with it. And if you’re lucky (or better yet skilful), when the NRE dissipates, you still have a good, developing relationship with the new person and a strong, happy, healthy relationship with your established partner(s).