Basically Web apps packaged in such a way that native APIs can also be exposed as browser APIs.
The Chrome/ChromeOS teams started with it, and Microsoft adopted them to UWP as well.
Safari also kind of support them.
https://developers.google.com/web/progressive-web-apps/
https://developer.microsoft.com/en-us/windows/pwa
Plenty of talks at Google IO and BUILD.
For example extension methods or suspend functions.
Hence why there is a guide on how to write Java friendly APIs from Kotlin.
Jetpack Compose is an interesting story, looks like it sprung into existence as response to Flutter, without any concrete ideas on how tooling should look like and total disregard for the ongoing efforts to sell constraint and motion layout editing on the graphical tooling to android devs still using the old layouts.
Having watched all SwiftUI tooling related talks, and from experience how Android Studio has evolved, I don't have great hopes that by Google IO 2020 we will see demos at the same level as WWDC 2019 SwiftUI ones.
As an Android developer I am kind of wary of how they push Kotlin for Android. I'm sure it is good, and I enjoy learning a new languages, but I already have a code base that I am worried they are going to make me rewrite.