Magnetic sensors are the most common type of sensor used in security systems.  They are often referred to as "contacts" or "switches."  They are usually discretely placed on access points, such as windows or doors, so that they are not visible from the outside. 

Once somebody brick / open your windows or door, the magnetic sensor is activated and the alarm will Arm. 
The best of this wirless magnetic sensor is there is no fussy wiring.. Just plug plug and work with your main wireless alarm (can get from our store as well).