Service Set Identifier
A Service Set Identified, or SSID for short, is used with 802.11 wireless network technology. Technically, to join a wireless network (or more specifically, to connect with a WAP or other piece of Wi-Fi hardware that drives a network), you must know the SSID. Most of the time the SSID is broadcast out unprotected, but not in all cases. When scanning for wireless networks, when results appear on screen, what you would perceive as the "name" of the network; that is generally the SSID.
If the SSID broadcast is not turned on the in the hardware settings, then it may get reported as an unknown network or might not even appear at all. Some people turn off SSID broadcasting as they feel like it gives them a level of protection against hackers. While it seems logical, it is not that hard for a determined hacked to use packet sniffers and other tools to find the SSID rather quickly. Then they could launch an attack on the wireless network protection itself (WEP is particularly vulnerable).