Cron is a daemon that executes commands at specified intervals. These commands are called "cron jobs." Cron is available on Unix, Linux and Mac servers. Windows servers use a Scheduled Task to execute commands.
For a modest personal site, you might set up this cron job to run once a day. For a more active site, you might want to run that job more often - perhaps every few hours or every hour.
Each e107 version has a different URL, you must request for cron to trigger. For e107, the URL contains a token, which is a "secret key" to prevent external abuse.