In layman's terms, the AP is not advertising its presence by broadcasting management packets containing the network name (ESSID) and other information. Also known as 'cloaked', or 'hidden'.
The ESSID can still be derived from data packets with no issue (just at a different offset), but if there is no activity on the network it cannot be detected by packets. All this serves to do is prevent devices without prior knowledge of the ESSID from automatically joining the network; it adds no more security than adding a blank cover-sheet to a document does.
(Last edited by aoz.syn on 9 May 2008, 18:31)