What are Core Web Vitals
In short
Core Web Vitals are three metrics Google uses to measure how good a site's experience is: loading speed (LCP), responsiveness (INP) and layout stability (CLS).
LCP measures how fast the main content appears. INP measures how fast the page responds to a click. CLS measures how much elements jump around while loading.
Together the three tell how smooth and comfortable a site is to use.
Why Core Web Vitals matter
Core Web Vitals are an official Google ranking factor. They are not the most important one, but between two otherwise equal pages the faster one wins. They also affect conversion directly.