What is the C wire and why is it required for the RET97B5D thermostat?

The C wire, or common wire, brings 24 VAC power to the thermostat from the heating/cooling system and is the primary power source for your thermostat. Without a C wire, your thermostat will not power up. If you don't have a C wire, you can look for an unused wire pushed into the wall and connect it to C after verifying it connects to the 24 VAC common at your heating/cooling system.