Here is very good explanation of First Input Delay ie. FID – https://web.dev/fid/
In short, it is a metric to calculate the time taken by a webpage to respond after a user does any kind of interaction with the webpage, it could be tap, scroll, keypress anything.
The following diagram shows the time taken by the main thread to respond after it receives user input. It should be less than 100ms.
Read more about FID – https://web.dev/fid/