The document discusses web workers, which are JavaScript scripts that run in the background independently of other scripts to avoid blocking the UI. Some key points covered include: - Web workers allow computationally expensive tasks to run off the main thread so they don't slow down the page - Features available to web workers include XMLHttpRequest and timers, but not the DOM or window/document objects - There are different types of web workers: dedicated workers run per page, shared workers can be accessed by multiple pages, and service workers act as proxies for network requests - Common uses of web workers include offline content, background syncing, push notifications, and making sites work as progressive web apps (PWAs)