Cron has 5 fields: min hour day month weekday.
*means “every”0 9 * * 1-5→ 09:00 on weekdays*/15 * * * *→ every 15 minutes0 0 1 * *→ first of the month
Write it left→right: When in the hour? → Which hour? → Which day? → Which month? → Which weekday?