It’s a shortcut for repeated access, and its whole risk is staleness: fast only helps when the saved copy is still correct.
Think about keeping the forms you use every day on your desk. You could walk to the file room each time a client asks, but that’s silly, so you keep a copy close by. That desk copy is the cache. The catch: if the form changed yesterday and your copy is from last month, the shortcut becomes the problem.
How it shows up
Your browser caches images, scripts, and pages so it doesn’t download them again. A server might cache a database result instead of repeating the query for every visitor. A CDN caches site files closer to users. In AI work, prompt caching reuses part of the processing when the same long context appears repeatedly, so the next request is cheaper or faster. Caching also connects to rate limits: reusing a saved result means fewer calls to the outside service. The hard part is invalidation, the plain question of “when do we throw away the desk copy and get a fresh one?” Cache too little and everything’s slow; cache too much and people see old prices, old permissions, or old files.
Why you care
Cache bugs are confusing because the system looks like it ignored you: you changed a logo but the old one still appears, you fixed data but the dashboard shows the stale number. The move is to ask, “Am I looking at the original, or a saved copy?” That one question explains a lot of strange behavior. Speed is only useful when the saved copy is still trustworthy.