The primary function of a thermostat is to regulate temperature in a given environment, maintaining a desired comfort level. It achieves this by controlling heating, cooling, or ventilation systems to adjust the indoor climate. Thermostats can be mechanical or digital and often feature programmable settings that allow users to set specific temperatures for different times of the day. By monitoring the air temperature, a thermostat signals the HVAC system to turn on or off as needed, promoting energy efficiency and comfort in homes and commercial spaces.