Even Drupal has to take out the trash and wash it's windows every once in a while. Ok, that sounds a bit silly, but what I'm talking about are those routine items which need to be carried out on some type of frequency - to keep your Drupal running smoothly.
It's Drupal's cron.php script which needs the love. It sits at the same level as the index.php script and it takes care of some of the "behind the scenes" stuff. It's an important aspect to your site and knowing how to configure and set this up is what this video is all about.
From covering a normal Cron setup (according to drupal.org) and using Poormanscron, as well as discussing Multisite cron, of which my current favorite is a simple shell script, this video covers as much as it can to help you gain a full understanding.
There are all kinds of other tips and tricks related to setting up cron, but essentially, you simply need to know that cron.php must be accessed on some type of frequency.