Stands for "Service Set Identifier."
An SSID is a name assigned to a Wi-Fi network. A network's SSID is a case-sensitive string up to 32 characters long, containing letters, numbers, spaces, and other special characters. Wi-Fi access points publicly broadcast their SSIDs to nearby devices to advertise an available connection. If multiple Wi-Fi networks are available in one place, unique SSIDs help someone select the correct network.
Wi-Fi routers and access points ship configured with a built-in default SSID, but a network's owner can change it. Default SSIDs often include the manufacturer's name with a random series of numbers and letters, making them technically unique but easy to mix up when several with similar names are nearby. If a network's SSID changes, every other device will see it as a new network and needs to reconnect to it manually.
A Wi-Fi network owner can also hide the SSID, which stops the router or access point from broadcasting its SSID. It will still have an SSID, and anyone trying to connect to that network still needs to enter it along with the network's password. Even if a network's SSID is hidden, every data packet sent over that wireless network still includes it to direct it to the right destination. Therefore, anyone actively scanning network traffic can still identify the hidden network's SSID.
NOTE: In addition to the network's SSID, Wi-Fi access points also broadcast a BSSID (Basic Service Set Identifier) that uniquely identifies the networking device itself. In most cases, an access point's BSSID is its MAC address.