Clear cache corresponds to a dump Magento Cache (cache config ,full page caching ... ), that is to say the removal of certain data stored in the cache server.
In a normal environment , Magento performs calculations to construct the HTML code of the page and puts on this occasion , a calculated maximum number of items in its cache . So -for the next visit of the same page , the most of the work Magento is simplified (and accelerated) . However, during the clear cache , the elements of the page are recalculated (typically when the base product has changed , or when a back-office user has decided to manually clear up the Magento cache ) .
Find how caches can affect your performance (rétablir liens hypertexte vers: The effect of your actions in the Magento back office)