If you have a compatible Prestige Thermostat, AND the installer code, you can attached this wireless sensor in a place in your home where the HVAC is less than optimal. It is completely wireless, runs from two small batteries, supplied with a new model.
You then link it to the thermostat via Honeywells proprietary redlink protocol. You do this through the thermostat and then press the connect button on the underside of the sensor. It does NOT use your home wifi or Internet to communicate with the thermostat.
Once it is paired, the temperature will be the averaged between the thermostat and the sensor. Thats the average, not the median. So if the thermostat is set to 74f and the temperature at the sensor is 71f, the temperature will be approx 72.5f in both rooms.
You can add up to 6-sensors per thermostat, and it will average across all of them. The sensor itself has only a light to indicate battery condition, it will turn red when it is likely to run flat within a couple of weeks.