A Greasemonkey Script is a small piece of JavaScript code that allows users to customize the way web pages look and function in their browser. It is primarily used with the Firefox browser, enabling users to modify existing websites by adding new features or changing the layout. These scripts can enhance user experience by automating tasks, removing ads, or improving navigation. Users can easily install and manage these scripts through the Greasemonkey extension, which provides a simple interface for creating and editing scripts tailored to individual preferences.