gpg error fix

[Verde Denim]

I setup Ubuntu 12.04 on a new machine today and in the course of updating
apt got a GPG error for ppa.launchpad. After some googling, I was able to
find this fix -

1) gpg --keyserver keyserver.ubuntu.com --recv <last 8 digits of key>

2) gpg --export --armor <last 8 digits of key> | sudo apt-key add -

But wanted to know why this works. A brief explanation would be most
helpful; thanks.
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