Lock COM - probably the ground wire, or could be the common connection on the relay
Lock NC - Lock, Normally Closed, probably a connection to the relay for the alarm
Lock NO - Lock, Normally Opened, same as above, just open
Normally opened and normally closed refers to the relays state with no power applied to it.
The common is connected to the NC in its power off state. So whatever is connected to common flows to the NC terminal. When power is applied the relay connects to he NO terminal and what is connected to the common will then flow to and be connected to the NO terminal.
Hope that helps out.
