Cache is a technique to speed delivery of web pages by saving portions of pages as they are generated. Then those "cached" portions are reused later when the page is requested again.
There are times when you will need to manually "clear cache" due to changes made in the TYPO3 Backend.
There are two clear cache options. Clear Configuration Cache is used rarely when you change the TYPO3 configuration. The one that matters most is Clear Page Content Cache which clears the cache for your webpages.