Time to First Byte (TTFB)
How long the browser waits before the first byte of the answer arrives. Good is 800 milliseconds or less, and everything after it inherits the delay.
It is not one number but a chain: redirect, DNS, connection, TLS, and only then the server's own thinking time. A site that looks slow at the server is often slow at the redirect — a hop from the bare domain to www spends a full round trip before anything has been asked for.
It is not a Core Web Vital, and it is worth watching anyway, because it sits underneath one. LCP cannot be better than TTFB plus the time to fetch and draw the largest element, so a slow first byte puts a floor under the score that no image tuning can lift.
Caching is what usually moves it. A page served from cache answers in the time it takes to reach the nearest node; a page rendered per request answers in the time your database takes, and the difference is visible in the field data long before anyone complains.