http://www.adobeforums.com/webx/.59b7c503 has the explanation.
However, I can give you a short version -
These forums are being migrated to new forum software, which should make you happy.
The name you use here cannot necessarily be used there (never mind why, just take it as read). You'll need to use your Adobe ID name (the name you used to register your software) to log into the new forums instead. If you don't have such an Adobe ID name, you'll have to create one.
The new system needs to be able to match up your Adobe ID name with your forum name from here, so that previous posts from here imported into the new system can be displayed under your Adobe ID name, the name you'll use in the new system. That's why, having logged in here, you were asked to say what your Adobe ID name is, for the system's future reference.
If you choose not to provide the information, your old posts will be shown as having been submitted by "Guest" because the new system will not know otherwise.
Here's an example.
My forum name here is Ozpeter.
My Adobe ID is Goodlooking.
I've provided the requested information (to say that forum user name "Ozpeter" is the same person as Adobe ID name "Goodlooking"). So when you read this post in the new forums, it will be shown as having been posted by user "Goodlooking". And if I log onto the new forum using "Goodlooking" (as I must, that being my Adobe ID), and add another post to this discussion, it too will be shown as having been written by "Goodlooking" as well as this post. So everything will tie up neatly.
(For a short version, that was, admittedly, long).