'Service Worker' is a type of script executed by your browser in the background, independently from a web page, paving the way for functionalities that don't require a web page or user interaction. Its current capabilities include features like push notifications and background synchronization.

Service workers primarily function as network proxies. They intercept network requests and manage resources by either caching them or retrieving them from the cache, subsequently delivering these resources to the web page. This ability facilitates effective offline experiences, periodic background syncing, and push notifications.

A vital attribute of a service worker is its capability to cache a web application's assets. Once a user accesses a web page, the site's assets, such as images, CSS, and JavaScript files, can be stored and then served from the cache during future visits. This caching significantly boosts performance and reliability, especially in areas with limited connectivity.

Service workers are crucial for the functioning of Progressive Web Apps (PWAs), as they enable these web applications to load swiftly and offer offline features. Operating in the background and independently from the app, they can execute tasks even when the app is not in active use.

To summarize, service workers are a major breakthrough in web technology, enhancing the reliability, speed, and user engagement of web applications. They are indispensable in the development of modern web applications, especially those striving to emulate a native app experience.