Use it if you wish to be notified (periodically) about missed calls, SMS or MMS.
Move the wheels to set the time interval in hours, minutes and seconds respectively. Tap the switch to turn it on/off.
Reminder acts in the same way as device ringer mode (sound, vibrate or mute), e.g. if device is set to vibrate, Reminder will vibrate but will not play sound.
Some third party SMS applications can prevent other applications to read from SMS inbox. If Reminder doesn't work, try switching to default SMS app and test it with a new message.