A very well-known cross-platform GUI library. If you google qt, the top hits are the official website and Wikipedia. Without clicking any link, the descriptions would have given you an answer.
I know what it is, it's just not lightweight.
It is compared to Electron; I don't know how WPF compares though, IIRC the 'de facto' way to make windows native apps nowadays also involves some web technology?
I don't think "de facto" exists on Windows anymore, but most WPF and UWP apps are 100% C#.