SSID full form is Service Set Identifier. It is a wireless network name that will be required when you want to connect to the WiFi. Depending on the configuration of the wireless network whether you will need to enter the password or not. Service Set Identifier consists of 32-byte alphanumeric name. SSID can be visible or hidden (for security purposes) depending on the requirements.
SSID can consist of several Access Point (AP).
Where you will find SSID
SSID on laptop
SSID on smartphones