Keep a copy nearby
A browser may save a website’s logo so it can show it again without downloading it. That saved copy sits in a cache. A server might also save a page or a recent result instead of producing it again for every request.
Caches can live in your browser, on a server, or on a CDN closer to visitors. Redis is one tool for keeping a server’s cached results in memory.
Copies can get old
If the original changes, a cached copy may show outdated information. A system can set a time to live for the copy or remove it when the original changes. This is why clearing your browser’s cache sometimes fixes a page showing old content.