It's a phone accessory. It lets you see and respond to things like SMS messages, caller ID, calendar reminders, etc. without taking your phone out of your pocket.

Kind of a small thing, but actually pretty handy when you're walking around or in a meeting or other places where glancing at your wrist is easier or less intrusive than using your phone.

(Plus, it tells time.)

This review goes into more detail: http://thewirecutter.com/reviews/best-smartwatch/