Each unexpected layout shift (a button moving as a banner loads, text reflowing as a font swaps in) is scored by impact-fraction × distance-fraction. CLS is the worst session-window sum over the lifetime of the page. Google's threshold is 0.1 for Good and 0.25 for Poor.
The fixes are mechanical. Reserve space for images and embeds with explicit width and height attributes (or a CSS aspect-ratio). Preload web fonts so a fallback font swap doesn't reflow your H1. Don't inject content above the fold after the first paint — push it below, or use a placeholder that occupies the final dimensions.
User-initiated shifts (a click that opens an accordion) are not counted. CLS measures the shifts the user did not cause.