A cache is temporary storage that keeps a copy of a page's elements, images, stylesheets, scripts, close to where they'll be used again, so a browser or server doesn't have to fetch everything from scratch on every visit. The first time someone opens a page, all of it has to load fresh. Cache it, and a return visit can pull much of that from a much faster local or nearby copy instead.
There are several layers of caching that stack on top of each other: browser caching on the visitor's own device, server-side caching that keeps a rendered version of a page ready to serve, and CDN caching that stores copies closer to the visitor geographically. Each layer shaves time off the load, which matters directly for Core Web Vitals and for how patient a visitor is willing to be. The tradeoff is freshness: a cached page can go stale if the underlying content changes and the cache isn't invalidated, which is why cache settings usually include an expiration window rather than caching forever.
