A door jamb switch is a crucial component of a door alarm system. It consists of a magnetic or mechanical switch placed on the door jamb and a corresponding part on the door strike plate. When the door is closed, the magnetic field between the two components is complete, indicating to the alarm system that the door is closed. Upon opening the door, the magnetic field breaks, triggering the alarm system to activate. This simple yet effective mechanism provides a reliable way to monitor door openings, ensuring the security of your home or business premises.