The Nest Temperature Sensor is a small plastic puck that connects to your smart thermostat over Bluetooth and, measures and reports the surrounding temperature. There's a tiny hole on the back that you can use to mount the sensor to the wall, or you can simply place it on a shelf — either way works.
Once the Nest Temperature Sensor is set up, you can check the temperature of the room it's in from the Nest app, and adjust your thermostat if needed. This is ideal for multiple story homes, where upstairs will usually be a few degrees warmer than the bottom levels.
If you tend to spend certain times of day in different parts of the house, you can also use the Nest Temperature Sensor to schedule your thermostat based around those habits.