If you visit a WordPress website that isn’t using cache, the website server reads the files in the WordPress installation and fetches the data stored in the database – posts, settings, and so on. It then constructs a HTML file from that data and serves it to your browser. Reading all of the data, and building the HTML, takes both time and processing power, and without cache, it’s repeated every time the website is loaded. If a website has multiple visitors at a time, the server can be bogged down and load times reduced.
If that same website was using a caching solution, the HTML would only be only generated on the first visit to a specific page, and then stored on the server. The next time a visitor comes to that same page, the server gives the visitor the HTML file stored on the server from the previous visit to the page. This results in faster load speeds for the visitor, and in less strain on the server. The cache is refreshed when the content of the page is updated, or when a specified amount of time has passed.
WordPress doesn’t have a caching solution built in, but there are numerous plugins that offer that functionality. One of the most popular caching plugins is WP Super Cache, which is developed by Automattic and used on WP Glossary.