My old thermostat did not have a c-wire, which is needed to power smart thermostats. Some, like this ecobee, come with an extension kit for such a situation. I was unsucessful in following those directions... I couldn't find half of the listed terminals in my furnace.
I had just about given up until I realized I had a spare wire in my wall that was unused, (the orange one!). I pulled it out and used the extension kit just to connect and power it. Viola! C-wire!