MediaWiki:Confirm-purge-bottom
Why purge?
If you have a page ('Page A') which is relying on information from another template or script ('Page B'), Page A will reduce the workload by remembering what Page B used to look like and working off an old version of it for a few days.
This can get annoying if you know Page B has changed and you want it to impact Page A now. You can force Page A to take another look at Page B by editing Page A, or you can save time by hitting 'purge'.
Please note that 'purge' is literally skipping a safety check which is there to stop the site being overloaded, so only use it when necessary and try to avoid it completely when the site is busy.