The wireless sync dongle is the smaller USB device that comes with the Fitbit Force, Flex, and One trackers.
The wireless sync dongle should be plugged into your computer to link your tracker with Fitbit Connect (which can be downloaded here).
Once your tracker has been paired to your Fitbit account, the wireless sync dongle is used to sync your data to your online Dashboard and update your tracker. The wireless sync dongle must be plugged in if you would like to sync using your computer.
For more information about syncing, see How do I get data from my tracker to the website? To get set up, see How do I set up my Fitbit tracker on a computer?