A cache keeps a copy of something that has already been fetched or worked out, so the system does not have to do the work again. When you revisit a page and it appears almost instantly, a cache is usually the reason.
The trade-off is freshness. A cached copy can be slightly out of date, so well-built systems decide how long a copy stays valid and refresh it when the original changes. If you ever see an old price or an outdated figure that corrects itself after a reload, the cache is the first thing a technician will check.