A cron job is something the server runs automatically at a certain time. Like clearing something out or sending mail.

 Lets look at one:

0 0 * * * /usr/bin/php -q /home/youraccount/public_html/periodic/cmd.php

     The first part is the time it should run. The first number is the minute, the second is the hour, the third is the day of the month, fourth is the month and the last is the day of the week. So this job would run on the "0" minute of the "0" hour which is the stroke of midnight. The "*" means "every".. so it will run every day of the month, every month, and every day of the week. Make sense? Then the "/usr/bin/php" is the path on your server to the php executable file. Not YOUR php file.. this is the executable everyone shares (if you are on shared hosting). Then the last part is the path to the file you want to run. It is ESSENTIAL you put a working email in the box above the cron jobs, that email is where your server is going to send any error messages.. with out it you won't know if your "crons" are working!