Every time you load the frontend of your WordPress site makes many, many database calls. WordPress pulls together many different assets and compiles them into webpages.
This is really time consuming and can make your site run slowly.
WP Super Cache is a great way to make your site run more quickly.
WP Super Cache generates static HTML files from your dynamic WordPress site. Those static HTML files will be served to the vast majority of your users. None of those database calls or compiling tasks are needed.
Here's your 10 minute guide to understanding and using WP Super Cache.