I'll describe NA the way one of my editor friends described it to me:
New Adult is where the protagonist/narrator of the book is in the late teens-early 20s, so more "college aged."
NA was meant to fill the gap between YA and adult novels, because with YA, the protagonist/narrator ages range from 15-17 and adult books usually have protagonists that are in their late 20s, early 30s and up. It was all about becoming an adult, learning new experiences as an adult, and having the age of an adult but not the experience and dealing with it. However, in mainstream/traditional publishing, NA has kind of been narrowed into one category: romance. It never really expanded to other genres, I'm not really sure why, but there aren't really any NA fantasies/mysteries/thrillers in mainstream right now. If an author wants to write an NA book (like myself) then if it's not romance, it's going to be a pretty hard sell.
Ava Jae, a published author and vlogger, released a video called "What Happened to New Adult"
https://www.youtube.com/watch?v=0j5b60KUbsE that kind of explains what happened to this age rating that didn't really take off. It's a shame, because while I personally still love YA books, I feel like I'm growing out of them and need something more, but I'm still not entirely interested in Adult books yet.