Stands for "Service Set Identifier." An SSID is a unique ID that consists of 32 characters and is used for naming wireless networks. When multiple wireless networks overlap in a certain location, SSIDs make sure that data gets sent to the correct destination.
The SSID is different than the name that is assigned to a wireless router. For example, the administrator of a wireless network may set the name of the router, or base station, to "Office." This will be the name that users see when browsing available wireless networks, but the SSID is a different 32 character string that ensures the network name is different from other nearby networks.
Each packet sent over a wireless network includes the SSID, which ensures that the data being sent over the air arrives at the correct location. Without service set identifiers, sending and receiving data in a location with multiple wireless networks would be chaotic and unpredictable to say the least.