What is SSID Broadcast ?
SSID (service set identifier) is a function performed by an Access Point that transmits its name so that wireless stations searching for a network connection can 'discover' it. It's what allows your wireless adapter's client manager program or Windows XP's built-in wireless software to give you a list of the Access Points in range.
Having SSID broadcast disabled essentially makes your Access Point invisible unless a wireless client already knows the SSID, or is using tools that monitor or 'sniff' traffic from an AP's associated clients.
Using the default SSIDs poses a security risk even if the AP is not broadcasting it, here are some standard ones that can possibly be probed by potenital attackers:
RoamAbout Default Network Name (Lucent/Cabletron)
Note that turning off SSID broadcast does not effectively protect the network from attacks, as network monitoring tools like Kismet and airodump-ng can still easily find the SSID, often within minutes.