Cache is the process of saving a website locally so the user doesn’t need to load it every time they visit the website.
This won’t increase the speed of your website the first time a user visits, but once they have visited it once they’ll experience the site much faster.
There’s a lot of great options out there for Cache.
As we use WP Rocket for a lot of other features, that is also our go-to for cache.
A lot of hosts also provide their own caching solutions. Siteground for example has it’s SG Optimizer plugin.