It's their version of a smoke and carbon monoxide detector. http://www.nest.com/smoke-co-alarm/life-with-nest-protect/ It ha a couple nice features. If you have a Nest thermostat, sensors in Protect will detect activity and assist in auto away. If it detects carbon monoxide, if the home has a oil or gas furnace, it will have the thermostat turn the furnace off. A wave will hush it if you get a little too much smoke when cooking, and it will warn you in advance of the alarm going off. Not available yet, but will retail for $120, battery or hardwired.