Each time when a visitor browses a webpage from your website, the full page will be rendered - block by block. This costs time, while most of the pages remain the same for each visitor as there are no visitor unique elements on it. Therefore it should not be necessary to render the full page again and again, but render it just one time, capture the result, and reuse this result for any future visits. This technique makes the loading of a webpage incredibly fast, and is called 'loading a page from the cache'. 

A result from the cache becomes outdated when anything relevant to the cached page is changed. For example by changing or adding a menu item or image to a page. If this happens, the cache invalidates and all related pages are cleaned up from the cache. We discovered an issue where the cache was invalidated more often than necessary. As caching is a complex feature, from version 430 on we started to investigate where, when and how often the cache is cleaned up, and when possible to fix the underlying cause. We from Webanizr want every website to be fast!