Cumulative Layout Shift measures instability. Every time a visible element moves without the user causing it, the browser records a shift score built from how much of the viewport was affected and how far things travelled. The metric reports the largest burst of shifts within a session window rather than a running total.
Google's threshold is 0.1 or below at the 75th percentile. Anything above 0.25 counts as poor.
Almost every real cause is unreserved space. An image without width and height attributes collapses to zero height until it loads and then pushes the article down. A cookie banner injects itself above the fold. A web font swaps in with different metrics and reflows the paragraph. An ad slot fills late and shoves the button the visitor was about to tap.
The fixes are equally concrete. Set explicit dimensions or an aspect ratio on every image and embed, reserve the space ads and banners will occupy, preload fonts and match fallback metrics, and never insert content above existing content unless the visitor asked for it.