When you use your computer and visit a website, it downloads that page to your computer temporarily. It stores those temporary files (pictures, video, text, etc.) in a cache. If you leave and come back to that page in the very near future, instead of your computer going online and fetching the data from that website, it will just fetch the temporary files that are stored in the cache on your computer. It’s much quicker, and much less strain on resources.
A website works in the same way, if it is setup and configured properly. A website is stored on a server. Other servers will fetch a website and store a cached version of it on their server. This greatly speeds up the delivery of the website, making it super fast.