Short answer: A PWA is a website that installs like a native app. Works offline, looks fullscreen, no app store needed.
Progressive Web Apps install from a browser instead of an app store. TenDollarPick is a PWA — on Android Chrome you'll see an 'Install' button; on iOS use Share → Add to Home Screen. After installing it runs offline (cached pages), looks like a real app, sends push notifications. The bridge between web and native.