After inserting a cron expression on fields above, you will receive a translation to plain english. I'm still trying to improve the translation. Please contact me with any suggestions you have. I have based the translation on the Quartz explanation.
Quartz definition of cron expressions: best explanation I found about it. But it is a little different than the cronjob app (it has a seconds field).
Wikipedia page about cron expressions: Wikipedia also has a good explanation