The white wire changes output when there is an alarm.
Normally hot means that in normal operation, the input power is coming out of the white wire, but turns off when there is an alarm. This is normally used in solenoid shut offs, so they slam shut when there is an alarm on the system, meaning that it is fail safe.
Normally cold means that the input power comes out of the white wire only when there is an alarm. This is most often used when hooked up to an external horn.