Cron jobs allow you to automate certain commands or scripts on your site. You can set a command or script to run at a specific time every day, week, etc. For example, you could set a cron job to delete temporary files every week so that your disk space is not being used up by those files.
Warning: You need to have a good knowledge of Linux commands before you can use cron jobs effectively. Check your script with your hosting administrator before adding a cron job.