Cumulative Layout Shift (CLS).

CLS measures unexpected layout shifts during page load. If your headline appears, then a banner pushes it down 200 pixels, that's a layout shift — and it's the cause of those moments where you accidentally tap the wrong button as a page is loading.

Google scores CLS from 0 (perfect) upward. Under 0.1 is "good," 0.1 – 0.25 is "needs improvement." The fixes are mechanical: declare width and height on every image, reserve space for ads or embeds, and avoid injecting content above existing content.

A high CLS score is one of the easiest things to fix and one of the most common things to get wrong.