A cache is a saved copy of information kept so it can be found again more quickly. It is like keeping a frequently used tool in a nearby drawer instead of walking to a distant cupboard each time.
Your browser may save parts of a website so the next visit loads faster. A service can also keep saved results rather than doing the same work again.
Saved copies can become out of date. Refreshing a page or clearing the browser cache may help when an old version keeps appearing, though most of the time the browser manages this for you.