High CLS = layout jumps as the page loads: an image appears without dimensions, a banner pushes content, fonts swap from fallback to primary. To lower CLS: always set width/height on img and iframe, use `font-display: swap` with a close fallback, reserve space for dynamic content.
CLS — Cumulative Layout Shift
CLS — how much the layout "jumps" during load. Target: ≤ 0.1.