What is a cache?
In one sentence: A store of answers kept nearby so they don't have to be worked out or fetched again.
A little more
Working something out or fetching it from far away takes time. A cache keeps a copy of the answer close by, so the next person who asks gets it instantly. Your browser caches pictures from sites you visit, and big websites keep caches in many cities.
The hard part is knowing when a saved answer is out of date. That's why caches give each copy an expiry time (max-age). A soft visit may keep showing an old logo until that timer ends, you hard-refresh, or the URL changes.
It's a bit like… Writing a friend's phone number on your hand so you don't have to look it up every time.