The C wire, often blue and sometimes labeled as COM or Com, is the "common" wire that supplies 24 VAC power to the thermostat. While many older thermostats operate on batteries and do not require a C wire, smart thermostats need it to power Wi-Fi and other advanced functions. If your old thermostat did not use a C wire, it might be hidden in the wall.

If you do not have a C wire at your thermostat, you have three options:

1. Run a new C wire from your HVAC control board.

2. Install a Meross C Wire Adapter on your HVAC control board. 

3. Install a 24VAC transformer directly to the thermostat's C and R terminals.