A cron expression in the form this site uses has five fields:
minute hour day-of-month month day-of-weekMinute runs from 0 to 59. Hour runs from 0 to 23. Day of month runs from 1 to 31. Month runs from 1 to 12. Day of week runs from 0 to 7, and both 0 and 7 mean Sunday.
Symbols
* means every value in that field. */15 in the minute field means every 15 minutes. 1-5 is a range. 1,15 is a list. 9-17/2 is every second hour from 9 through 17.
The Cron Expression Generator checks those forms and describes them. It does not install a crontab and it does not run a command. Copy the expression into the scheduler you actually use.
What is left out
Quartz and some other schedulers add a seconds field and tokens such as ?, L, W, and #. Those are rejected here so a five-field expression is not confused with a six-field one.
One more trap: when both the day of month and the day of week are constrained, schedulers disagree about whether both must match. Read the manual for the scheduler you will paste into.