NC indicates that the contact is normally close
NO indicates that the contact is normally open
definition
Contacts that are in the disconnected state under normal (no power) conditions are called normally open contacts.
normally-closed contact
Contacts that are in a closed state under normal conditions (no power, no current flow) are called normally closed contacts.
Normally open - NO (normal open) is usually disconnected, that is, the coil is disconnected without power.
Normally closed - NC (normal close) is usually closed, that is, the coil is closed without power.
application area
Normally open and normally closed contacts are an indispensable part of both contactors and relays, especially relays.