Caching's purpose is to help your site load pages and its content quicker.
Caching is a system that checks your site for updated content in a set frequency. Usually, every 15 minutes it will check for new content additions which it will then add to the page. So if you add something to a page and it does not show up instantly it is because it has not been “seen” by your site yet. The added content will show up after the next cache check.
So how does this help your site speed? If your site is looking for new information every 15 minutes instead of continuously every second it can load content quicker because it doesn’t have to take the time to check every item, each time it loads a page.