[lucideer]

As others have commented, you're thinking of WebWorkers, whereas ServiceWorkers are quite a new, and separate thing.

The way I've (over?)simplified explaining ServiceWorkers to people is that they're like a proxy application running between your browser tab and the actual web service (server).

They can see/intercept/manipulate requests and responses, and have their own cache. This obviously enables offline mode (the request goes through the ServiceWorker and can be responded to without contacting the web if there's a cache), but also potentially quite a few other features and ideas.