Ubuntu with newest version of KDE stuffed into it. In short it's more for developers to test latest KDE versions in a relatively stable and easily maintained environment but gained popularity since it's newer than Kubuntu and doesn't enforce snaps.

By "stable base" they mean Ubuntu. The package systems (apt and snap), the kernel choice and configuration, the background services are what make the "back end" of a distro and Ubuntu is an example where this backend is given LTS (long term support) maintenance and thorough testing, even if that also means using older versions of the backend software in question. KDE Neon uses this because if something goes wrong with the distro it's likely their fault and not some backend stuff unrelated to KDE. Ubuntu is also good for building spinoff distros as most of the work is done for you - You just make your tweaks to a "minimal" ubuntu ISO and share it!

GnomeOS is probably the main example of a distro designed for testing rather than daily use. It's maintained by GNOME for testing their changes and can also be used for daily stuff if you want. Not sure what else "quasi distro" could mean, but there are a bunch of Ubuntu-based distros (And spinoffs to other big distros) that exist on a spectrum of "almost identical" to "completely redesigned".

I used it for a few months. It's pretty good! You get brand new KDE features on top of a familiar ubuntu system. It gives you the feeling of a bleeding edge UI experience but you don't risk the software driving that new UI being super buggy or breaking things. That being said, some users (like myself) prefer different software managers or don't like Ubuntu for other reasons. KDE-Neon as a distro in itself is a pretty solid choice, though.