The two terminals are normally used to detect water: in case of a leak the water completes the circuit between the two terminals (located at the bottom of the sensor).
Since these screw terminals are easily accessible, you can also connect other things to it. The water sensor basically acts as a binary sensor: completed circuit = alarm on; incomplete circuit = alarm off.
Both the pressure mat and seat occupancy sensor have two wires that you can connect to the terminals. Then, when you apply pressure to the mat/occupancy sensor, the circuit completes and the alarm of the water sensor goes on. When you release pressure, the circuit is not complete anymore and the water sensor’s alarm goes off.
