The name of a particular Wi-Fi network.
Typically, Wi-Fi networks in a given area will broadcast their names (SSIDs). Devices nearby (within range) can collect a list of these network names and present that list to the user. The user can then choose form this list to select the Wi-Fi network they want to connect to.
Some networks do not broadcast their SSID and therefore will not appear in lists of nearby networks. But users may still be able to connect if they know the SSID and enter it manually on their device.