A cron expression is a string of five space-separated fields that defines a recurring schedule: minute (0–59), hour (0–23), day of month (1–31), month (1–12), and day of week (0–6, where 0 = Sunday). Servers use cron jobs to run scripts, send emails, take backups, and automate any recurring task at precise intervals.
* means "every value"; / defines a step (*/5 = every 5 units); - defines an inclusive range (1-5 = values 1 through 5); , separates a list of values (1,3,5). For example, "0 9 * * 1-5" means "at 09:00, Monday through Friday", and "*/15 * * * *" means "every 15 minutes".