Much of what makes software feel fast is not doing work at all, but remembering the answer from last time. A cache stores the results of slow operations so that a repeated request can be served instantly instead of recomputed or fetched again.
The art is knowing when a stored answer has gone stale and must be refreshed, since serving outdated data can be worse than being slow. Caches appear at every level, from the processor to the browser, quietly saving most systems from doing the same work over and over.