The term rules refers to the conditions for creating contact-based email reminders.
Why do we need rules at all? Such conditions give us great flexibility to create new email reminders depending on any fields in contacts, and not just on certain specific predefined fields.
The rules page lists all your rules. You can easily edit, delete or run certain rules.
Typically, rules should be run once a day to create new email reminders, depending on the date(s) in contacts and current day.
Of course, this can be more often if the conditions depend on the time of day relative to the current day.
Or rarely, if the conditions are not dependent on dates and only on some other fields. In this case, the conditions may depend on how often new contacts are added to the system.