They (probably) refer to the authorization ID and password.
Most DRM books are authorized using your Adobe ID.
Goto menu/Help/Authorize Computer (Library/Authorize Computer on older more reliable ADE v1.7.2), (ADE == digital editions)
and enter your Adobe ID and password; the same ones you use for logging on to the Adobe web site.
(Except the Adobe web site allows special characters in the password and ADE doesn't,
so if you have those log in to Adobe at the website and change to a weaker password).
Some book vendors use the Adobe DRM mechanism but with their own custom id/password (? user name/code).
You need to check with the vendors how they organize their IDs.
I think Google uses the usual google login credentials,
and I have heard some use the email(ID/user name) and credit card number (password/code) you used when buying the book.
See Custom User ID in http://www.adobe.com/uk/products/digital-editions/features.html
and in the Getting Started eBook installed with Adobe Digital Editions.