Talking about What is Cache, Cache in simple terms is just a data that stores on your computer, mobile etc. Caches are created automatically by bt browser. It improves the website loading speed if you are a returning visitor.
You must have observed like if you open a website for the first time it takes time to load and if you open that website again the loading time reduces. This is because of Cache Data. Cache data store data of your website on your system and helps to load faster when the user visit is again.
If you are using a WordPress website, your website loading time will be more because as compared to simple HTML website WordPress has a lot more files on the server.