FID stands for First Input Delay. It is a web performance metric that measures the time (in milliseconds) from when a user first interacts with a page such as clicking a link, tapping a button, or using a custom JavaScript-powered control to the moment the browser is actually able to respond to that interaction. FID is important because it reflects the real-world experience of users trying to interact with a web page. A low FID ensures that a site feels responsive and interactive, which is crucial for user satisfaction and engagement.