This is a question we are often asked. A cache is basically a hard copy of the pages of your website, let us explain:
WordPress is a database-driven content management system. In other words, your site does not actually have physical pages, when a user clicks "About Us" or any other link to a page on your site WordPress calls to the database for all the information needed to build that page, puts it all together then shows the page to the user.
The issue with that is it takes time. These days Google and web users, in general, want pages to be FAST!
So what a cache does is builds actual pages based on the info in the database. So when a user clicks "about us" rather than WordPress talking to its database and building that page every time it is called, WordPress looks at the cache folder and sees the page has already been built and shows that instead. Much faster.
All you have to bear in mind with a cache is that if you edit a page or approve comments when you are done you should clear the cache then rebuild it.
Our servers run something called Litespeed which takes care of basic caching at a server level. If we build your site for you we may also use other caching plugins there will be help in this knowledgebase on using specific caching plugins.