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. This regular visit will tell Drupal to perform its periodic tasks, and this will help keep your system running smoothly.