You can generate the reminders in one of two ways:
Both of these execute the rules you've defined for generating the reminders, but they don't actually deliver them. To deliver the messages, if any, use the E-mail Robots, the SMS Robot, or the Standalone E-mail Robot. The first two of these deliver just one type of message (e-mail or SMS, respectively), where the Standalone E-mail Robot can deliver both automatically.
If you do not have one of the robots running, messages are never delivered.