A token is required to install the Electronic Updates Windows Client, or to integrate against the REST API.
A token is a series of characters used to identify a user. The token is equivalent to the username and password which you use to access the Electronic Updates portal. By using a token for Windows Client or REST API authentication, you can continue to get uninterrupted access to automated data downloads even when a password is changed or an account becomes locked. Furthermore, it is not possible for a user to get locked out by submitting an incorrect token.
You can locate your token by logging into the Electronic Updates portal and selecting the Automatic Updates tab. However, as the token does not change unless explicitly reset (as explained below), once it is known, it is not necessary to look it up again.