(It means web pages that use JavaScript to gain access to some “app” features, like offline-functionality and the device sensors as shown in the blog post.)
Progressive web app. They're kind of like JavaScript-driven websites that think they're native apps. They tend to be characterised by their use of modern web standards to do some useful things, like being available offline, and caching everything heavily to cut down load time. It's like a website you can install, basically.