A programmable thermostat is a device that allows you to set schedules for your HVAC system to automatically adjust the temperature based on your preferences and routines. It works by communicating with your HVAC system to turn heating or cooling on or off at specified times. This helps maintain comfortable indoor temperatures while maximizing energy efficiency. By reducing the system's operation during times when you're away or asleep, a programmable thermostat can lower energy bills and reduce wear and tear on your HVAC system.