What Is a Cron Expression?
A cron expression is a string of five fields separated by spaces that defines a recurring schedule for automated tasks. It is used by the Unix/Linux cron daemon to trigger scripts, commands or programs at specified times. The five standard fields represent — in order — minute, hour, day of month, month and day of week.
For example, 0 9 * * 1-5 means "at 09:00 on every weekday", and */15 * * * * means "every 15 minutes".