First Input Delay (FID) is a web performance metric that measures the time it takes for a webpage to respond to the first user interaction, such as clicking a button or a link. It is an important aspect of user experience, as a quick response time can make a website feel more responsive and engaging. A low FID indicates that a webpage is ready to respond to user actions without noticeable delays. High FID values can lead to frustration, as users may feel that the site is unresponsive. Optimizing FID involves improving the loading speed and interactivity of a webpage, ensuring a smoother experience for users.