The relay is hardwired to the back of the heater and physically turns the power ON and OFF to the heater. The wireless wall switch sends a signal to the relay to tell it to turn the power ON and OFF. The switch needs to be within 80' of the relay and the signal can go through walls.
The relay needs to be located away from the heater and protected from the weather. It also needs to be accessible to change the timing, if required. The relay is normally located in the ceiling or a plastic electrical enclosure to protect it from the weather.
The relay also has a built-in Auto-Off timer that can be programmed to 30, 60 or 120 minutes. This is very useful so the heaters are not left on when you aren't using them.