What does NO and NC mean?

Normally Open (NO) and Normally Closed (NC) terms refer to type of dry contact or wet contact. Put very simply, a Normally Open sensor will have no current when in a normal state but when it enters an alarm state it will have +5V applied to the circuit. A Normally Closed sensor will have +5V applied when in a normal state but when it enters an alarm state it will have 0V applied to it.