A cache is a saved copy kept close by so a system doesn’t redo the same slow or expensive work. Your browser caches images so it skips the download, a server caches a database result instead of rerunning the query, a CDN keeps site files near visitors, and prompt caching reuses processing when the same long context repeats. The whole risk is staleness: fast only helps while the saved copy is still correct.
The forms you use every day are the everyday version. You could walk to the file room each time a client asks, but that’s silly, so you keep a copy on your desk. That desk copy is the cache. If the form changed yesterday and your copy is from last month, the shortcut becomes the problem.
Why you care
Cache bugs are confusing because the system looks like it ignored you: you changed a logo and the old one still shows, or you fixed the data and the dashboard keeps the stale number. Ask one question: am I looking at the original, or a saved copy? That explains most of the strange behavior. Speed only counts when the copy is still trustworthy.