Some of the calculations that are done when your site loads a page take a long time to run. To save time when these calculations would need to be done again, their results can be <em>cached</em> in your site's database. There are internal mechanisms to <em>clear</em> cached data when the conditions or assumptions that went into the calculation have changed, but you can also clear cached data manually. When your site is misbehaving, a good first step is to clear the cache and see if the problem goes away.