Google's three Core Web Vitals each measure a different piece of real user experience: LCP (how long the largest visible element takes to render), CLS (how much the page unexpectedly shifts around while loading), and INP (how responsive the page feels when someone actually interacts with it). All three came from research into what visitors notice and complain about, not arbitrary technical thresholds.
How to fix what it finds
A slow LCP is usually a large hero image or unoptimized web font blocking render — compress images, preload critical assets, and avoid render-blocking resources. Layout shift almost always traces back to images or ads without a reserved size, or content injected above what's already loaded. A weak INP usually means heavy JavaScript tying up the main thread when a visitor tries to click or type.