The term "SSID number" refers to a service set identifier, which is a 32-character number that is used to identify a specific wireless network. Its main focus is to ensure that data gets transmitted over the correct network when there are multiple overlapping networks.
SSID numbers can be especially beneficial in areas where multiple wireless Internet connections are used in close proximity. An example of this would be an apartment complex where each resident has his or her own network. The SSID ensures that the data is not mixed up between multiple people browsing the Internet at the same time.