A normally open reed switch consists of a glas tube with two reeds sealed inside it. The reeds are connected to an external circuit. If there is no magnetic field present, the reeds are unmagnetized. There is a gap between the reeds, so the circuit is not complete and no current can flow. If a magnet is brought near to the switch, the iron reeds are magnetized and attract each other. The circuit is complete and current will flow. Normally open reed switches can be used as safety switches.
If an extra non magnetic contact is added, the reed switch can be used as a normally closed reed switch. Current flows until a magnet is brought near, then the reed switch changes the point of contact to break the circuit and the current stops flowing. This is a normally closed reed switch, shown below.
Normally closed reed switches can be used as alarms.