The HTML cache is an output cache that caches renderings, not whole pages. You use the HTML cache to improve the performance of websites. Every time Sitecore renders a component on the page, it checks the HTML cache to ascertain if the cached output is present. If it is, it is retrieved and rendered immediately otherwise it goes through the normal process of running the rendering pipelines and constructing the output.

https://doc.sitecore.com/developers/101/sitecore-experience-manager/en/configure-html-caching.html