A Nike+ account is a free profile users create on Nikeplus.com that stores all data earned with Nike+ devices and experiences, such as the Nike+ FuelBand or Nike+ Running App.
If you get a new Nike+ device, you'll be prompted to log into your Nike+ account as part of the setup process. This ensures that all the data you record on that device is saved to the same Nike+ account. You can link as many Nike+ devices as you wish to your Nike+ account.
Storing all your activity data in your Nike+ account gives you the ability to manage your entire athletic life in one place.