The Service Set Identifier (SSID) is the name of your wireless network. By default, most WAPs broadcast the SSID. This makes it easy for users to find the network, as it shows up on their list of available networks on their wireless client computers. If you turn off broadcasting, users will have to know the SSID to connect. Some folks will tell you that turning off SSID broadcasting is useless because a hacker can use packet sniffing software to capture the SSID even if broadcasting is turned off. That's true, but why make it easier for them?
That's like saying burglars can buy lockpicks, so locking the door is useless. Turning off broadcasting won't deter a serious hacker, but it will protect from the casual "piggybacker"