A “common wire” provides continuous flow of 24V power to a thermostat. The continuous power enables the thermostat displays and Wi-Fi. These are three things you can do if you do not have a C-Wire. Install new thermostat wires that include the C-Wire. Because this is the most complicated option, think about hiring a professional for this job. Purchase and install an adapter. Choose a thermostat that includes an adapter. Choose a thermostat that does not require a C-Wire.