How a cache works
Without caching, every visit makes your server do the same work again: run the code, query the database, assemble the page, send it. A cache saves the finished result the first time and serves that saved copy to everyone who follows, until it expires or something clears it. The expensive work happens once instead of every time.
There are several caches stacked on top of each other, and knowing which is which saves hours of confusion. A page cache stores whole finished pages on your server. An object cache stores the answers to individual database queries. A content delivery network stores copies at locations nearer your visitors. The browser cache keeps images, stylesheets and scripts on the visitor’s own device so a second page view fetches far less.
Why caching matters
It is usually the single largest speed gain available to a content-managed site, and unlike a rebuild it costs almost nothing. Serving a stored page skips the database entirely, so time to first byte drops and everything the browser does afterwards starts sooner.
It also holds a site up under a spike. A shared server that would stall under a rush of visitors can serve the same cached page to all of them cheaply, which matters if a campaign, a news mention or a festival period sends more traffic than usual. For a mobile-heavy audience on variable connections, the browser cache is doing quiet work on every repeat visit.
Common mistakes with caching
The famous one is not realising a cache is there. You edit a price, reload, and see the old page — so you edit again, or conclude the site is broken. Nothing is wrong; a stored copy is being served and needs clearing. Always check the live page in a private window after publishing something important.
The opposite mistake is caching pages that must never be shared: a logged-in account page, a basket, a checkout. Serving one customer’s cached page to another is a genuine privacy failure, and it comes from over-eager configuration rather than from caching itself.
The third is stacking several caching plugins together in the hope of compounding the benefit. They conflict, and diagnosing which layer is holding a stale copy becomes guesswork.
How to act on it
Run one caching layer on the server, not three, and confirm that cart, checkout, account and any personalised page are excluded. Clear the cache after every content change that matters and after every release, then verify in a private window rather than in the browser you have been working in.
If pages are still slow with caching on, the cache is not your problem — look at server response, image weight and scripts instead, which is where page speed work actually pays off.