This document discusses service workers, which are scripts that run in the background of a web application to enable features like offline functionality, push notifications, and A/B testing. Service workers act as a proxy server to cache app assets, have access to the Fetch API but not the DOM, and can control client requests. Resources listed provide information on how service workers work, examples of sites using them like Pokedex.org, and Google's Workbox libraries for implementing service worker code.