Cumulative Layout Shift (CLS) measures visual stability: the largest burst of layout shifts during the page's lifecycle, where each shift is scored by impact (how much of the viewport moved) times distance (how far). Layouts shifting after the page appears stable cause users to misclick or lose their place.
Google's CLS thresholds at the 75th percentile: under 0.1 is "Good", 0.1 to 0.25 is "Needs Improvement", over 0.25 is "Poor". CLS is a confirmed Google Search ranking signal.
Common causes: images and ads without explicit width and height attributes, web fonts that swap and reflow text (FOUT), and dynamically injected content above existing content. Reserve space for every async element and CLS approaches zero.