I believe ServiceWorker is one of most important specifications for the next web world. Offline and its technologies are very friendly concepts to native application developers. But, now I think front-end developers have to know that for stepping into new paradigm. With ServiceWorker, you can make your web application can run offline, and it also means you can make your web application load extremely fast.
I've told about ServiceWorker very briefly in this slide. But you can understand how ServiceWorker runs on. If you want to know its usage, I highly recommend Topeka, which is a polymer demo application at google I/O 2014, that also includes material design and ServiceWorker in inside of it.
If you want to know ServiceWorker some more or in detail, I'd like to recommend to read the following, written by Jungkee Song, one of authors of this spec.