Service Worker
In plain terms
A background script enabling offline support and caching.
Going deeper
A service worker sits between the app and network, intercepting requests to cache responses, enable offline use, and handle push notifications. It runs separately from the page.
Under the hood
Event-driven and runs on a separate thread; the backbone of PWAs and offline-first strategies.
Examples
PWAs, offline caching, push notifications, background sync.
Think of it like...
Like a personal courier who caches your mail so you get it even when the post office is closed.