Disclaimer: always have a backup before doing any kind of optimisation to your website. Sometimes things break, so it’s important to have a save point to go back to.

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.

If you’d like a free option, the following are also good:
WP Fastest Cache
WP Super Cache
W3 Total Cache

A lot of hosts also provide their own caching solutions. Siteground for example has it’s SG Optimizer plugin.