This is explained at the bottom of the "Adding the Repositories" part
(https://help.ubuntu.com/community/Medibuntu#Adding the Repositories):
"
Then, add the GPG Key:
sudo apt-get update && sudo apt-get install medibuntu-keyring && sudo
apt-get update
You may be asked to accept this package even though it cannot be authenticated.
This is normal; typing "Yes" means you trust Medibuntu.
"
** Changed in: ubuntu
Status: New => Invalid
--
medibuntu could not be verified - NO PUBKEY
https://bugs.launchpad.net/bugs/291778
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