Service workers enable an application to run a background process that manages some lower-level features. This can include push notifications, syncing data in the background and even providing an experience for when the device is offline. The level of control is up to the developer.
Service workers have been around for some time on Android, but they have also recently made the jump over to IOS.
While the focus is on mobile devices, they also work on desktop. Now that the majority of visitors will have access to the functionality, there is no better time to start improving the experience for users. • w3.org/tr/service-workers-1