The SSID is the identifying 'name' of a wireless network. The
PSP can connect to wireless networks, you can set it to scan for
one, or if you already know the name, enter it manually.
Obviously the SSID will be different for every PSP, but the SSID itself is a property of the wireless network, not the PSP itself, you're just entering it into the PSP so it can recognise and access that network.