What is FCP?
First Contentful Paint (FCP) measures the time from navigation to the time when the browser renders the first bit of content from the DOM and is not considered a Core Web Vital.
Here you can view an example of when FCP occurs.
What is a Good FCP Score?
To provide a good user experience, sites should strive to have a First Contentful Paint of 1.8 seconds or less. To ensure you're hitting this target for most of your users, a good threshold to measure is the 75th percentile of page loads, segmented across mobile and desktop devices.
(Source: Google, https://web.dev/fcp/#how-to-improve-fcp)
Additional Information
For more information on how to improve your site's FCP, check out this article from web.dev.