A cron expression is a string of five fields separated by spaces that defines a schedule. The fields are: minute, hour, day of month, month, and day of week.
The asterisk (*) is a wildcard that means "every" value for that field. For example, * in the minute field means every minute.
The */N syntax means "every N units". For example, */15 in the minute field means every 15 minutes (at 0, 15, 30, 45).
No. All parsing and calculation happens entirely in your browser. Nothing is sent anywhere.