It's mentioned on the registration page:
RESTRICTIONS FOR NEW MEMBERS
After registering, you will be unable to post in any section except "newbies" until you have spent some time on the forum and have published a few posts.
If you are registering to ask a question, please ask it in the newbies section. Do not wait to ask it just because you must post it in "newbies": the question is very likely to have already been asked. If you don't end up getting good responses, you can ask it again elsewhere after you are established, or you can move the entire topic.
If you are commenting on Bitcoin, use your newbie wait time to read more about Bitcoin. If you are criticizing Bitcoin, find similar criticism using the search tool to see which points have already been covered. A good use of your newbie wait time is reading Satoshi's old posts.