It's a code needed to unlock a phone that's locked to a particular network.
If you're getting such a message when trying to use 48 then your handset is locked to another network provider and you'll have to get it unlocked before you can use it on 48.
See here:
If you need to contact 48 support then see the links in my signature below.
But they won't be able to unlock the handset for you.
