What exactly is an "Ubuntu App", as made with the Ubuntu SDK?

I think that what an "Ubuntu app" is is really written in the explanation, i.e. a program that can work on Ubuntu (both Desktop and Touch).

With that SDK you can create QML and/or HTML5 applications.

This means that they can work on all the desktops that support these formats. I personally do not know a list of desktops with such capabilities, apart from the default Ubuntu interface, Unity.

The SDK is built thinking of Ubuntu (it is Ubuntu SDK, not Kubuntu, Xubuntu, Ubuntu Gnome, Ubuntu Mate, ecc) and gives easy access to the services provided by Ubuntu (such as Content Hub, as explained in the Platform section).

I guess that it can be easily adapted to write programs for other desktops that take advantages of QML and HTML5.