SSID is the Service Set ID, which is used to identify the particular wireless LAN to be accessed. On many wireless LAN access point devices, it is possible to disable SSID broadcast. In fact, it is a good security idea to change the SSID from the default and to disable the broadcast. This does block tools like Netstumbler from finding your LAN. However, this alone does not make your wireless LAN totally secure. You should use some form of VPN for your connections between client and access point in order to ensure secure communications. A good reference paper is Security and the 802.11b Wireless LAN, written by Sean Griffin and posted on the SANS Institute site.
For more information on this topic, check out these resources:
Best Web Links: Wireless security
Best Web Links: Virtual Private Networks
Scheier's Security Product Round Up: Flawed wireless LANs inch their way to acceptance