The Microsoft Band app associates your data with a Microsoft account in order to store your data in the cloud.
A Microsoft account is an email address and password that you can use to access Microsoft services. It can be a Microsoft email address, like YourName@live.com, or another email address, like one from Gmail or Yahoo!
If you use a Windows Phone, Xbox, Office 365, Outlook, or any other Microsoft service, you’re already using a Microsoft account. If you don’t have a Microsoft account, it’s easy to get one, and it’s free. Just follow the prompts while setting up the Microsoft Band app.
Learn more about the advantages of a Microsoft account, how to create an account, and how to choose which account to use if you already have several accounts. See Microsoft account
If you’ve forgotten your password, see I can't sign in to my Microsoft account