Service workers let you decide what a user experiences on your website when there is, is not or is limited network connectivity.
The service worker is a game changer. It is no small thing; not because we can make web applications work offline but because we can fully control how our web application access the network. Control like that gives us the ability to make the web feel genuinely native in almost every sense.
This native-like web experience is the ethos of the Progressive Web App. Enabled by the service worker.
If you want to dive deeper these resources will get you started: