Okay so what is cron? Or what’s a cronjob?
Well if you search for it Wikipedia will tell you that cron is a job scheduler in operation systems other than Windows. Consequently a cronjob is a command or a script (basically a series of commands) with is scheduled to be executed at a given interval.
Now in Drupal cron is used for different tasks to maintain the system (search the Documentation for more information about cron).
The main thing you should know is: cron is very important for your search and other modules may need it to do what ever you want them to do (e.g. Rules can use cron to trigger some actions or the other way round, whatever you want to do…).
In Drupal 6 you have the choice between setting up cron with your hosting provider or using your computer or … using the module Poorman’s Cron (how to do one of the others? – search the Documentation). With module you can decide how often cron is run. When you use the module you will find the configuration on the page with the site information.
In Drupal 7 Poorman’s Cron was integrated into the core so you do not have to do anything else. You can find it under > Configuration > System > Cron.
Depending on how busy your site is/will be/ should be you can choose how often you need cron to run. Of cause you can also adapt your configuration. Up until now I didn’t have any problems with using an interval of 3 hours. But I didn’t have so big projects it may be different to you.