First Input Delay
Core Web Vitals and Google page experience signals play an important part in the SEO startegry. You can read the Google Page Experience overview if you want to get some idea about it.
What is the First Input Delay?
First Input Delay is the time between when the user interacts with a page, clicking on a link or button, and when the browser starts processing the event handler.
In other words, when a user does an action on web pages he has to wait sometime to see the response to that action.
This is used to measure the interactivity of the pages.
You have to measure this metric in real-time as you need the user to do action on the web page.