An SSID is a unique identifier that wireless devices use to identify and connect to a specific wireless network. The full name of an SSID is Service Set Identifier. An SSID can be up to 32 characters long.
Most home and small office routers come with a default SSID set by the manufacturer. This is usually something like Linksys or NETGEAR. Many people leave the default SSID in place because they don't think about changing it. That's not a good idea, however, because anyone who knows the default SSIDs for common router brands can easily access your wireless network if you don't change the default SSID.
When you set up a new router, one of the first things you should do is change the default SSID to something unique and not easily guessed. In addition, most routers allow you to enable what's called hidden mode for the SSID. When hidden mode is enabled, the router doesn't broadcast the network's name (SSID). Users must know the exact name of the network before they can connect to it.
Hiding your Wi-Fi network's SSID might seem like a good way to keep unwanted users from connecting to your network, but it's really not very effective. Even if you hide your Wi-Fi network's SSID, someone with basic knowledge of Wi-Fi networking can still find and connect to it fairly easily using readily available tools such as NetStumbler or Kismet. So hiding your Wi-Fi networks'SSIDs isn't really worth the trouble.