I just want to know what each digit means in (String CRON_EXP = '0 0 * * * ?'; ) AND Can you explain what does meaning for below lines.
System.schedule('Hourly', '0 0 * * * ?', new MyHourlySchedule() );
System.schedule('Start', '0 0 0 10 9 ? 2013' , new MyScheduleStart() );