How about doing the obvious and searching for that warning? You’ll learn about Secure Apt and PGP keys.
I did this and am did not find the correct answer.Long ago I installed debian-archive-keyring via synaptic, this solved all problems then, but lately I added another archive to pick up come codecs stuff and started getting those error messages again. I no longer suscribe to those so I have no problems now - and your answer would not help.
Would it not be better to tell what needs to be installed if you know it? Pointing to where the person could find more info is also extremely helpful but may add to confusion.