Basically, actionPollar is a timer that sends an AJAX request to the server according to a time interval that you specify. Each request can result in a full or partial page update.
An <apex:actionPoller> must be within the region it acts upon. For example, to use an <apex:actionPoller> with an <apex:actionRegion>, the <apex:actionPoller> must be within the <apex:actionRegion>.
Hope this helps!