A door contact is an electronic device that connects to a door and a door frame or window frame that is wired into the on-site security system.
Door contacts are small magnets that are installed on the door and the door frame (also seen on windows). When the door is shut, they are in contact with each other and let the alarm know that the door is closed. This allows the alarm system to arm. If the contacts separate from each other, it will trigger the alarm and the monitoring entity.