Whenever a user visits your website, their browser sends a request to the server in order to call all required data from the page. The server works on this request using its database and sends all necessary data and files to the browser. For large websites with a lot of – or particularly big – CSS files, images, or JavaScript files, this request can take quite some time.
In order to reduce this time, the server uses what is known as the cache. Instead of loading dynamic objects from scratch each time and calling them from the database, a static version of the site is stored in the cache. The user, therefore, receives a finished website, which may not be the most recent version but loads significantly faster. This is exactly what a WordPress caching plug-in can help you with.