Caching means keeping a ready-made copy of something so it can be handed over instantly the next time, instead of preparing it again from scratch.
On a website, building a page takes real work: the computer gathers the text, pictures and layout together each time a visitor asks for it. With caching, the finished page is saved after the first visit, and the next visitors get that saved copy straight away — so the site opens much faster. An everyday comparison: instead of cooking a dish from raw ingredients for every single guest, you prepare a batch once and serve portions immediately. The saved copy is refreshed from time to time, so visitors still see up-to-date information.
Good caching is one of the biggest and cheapest ways to speed up a site, and it is often set up on the Hosting where the site lives.