I think I know what a signature means in computerese, but I do not
know what to do when the following message is received at the end of
an apt-get update run.

W: GPG error: http://ppa.quickbuild.pearsoncomputing.net squeeze
Release: The following signatures were invalid: BAGSIG
43F1DC4F2B8638D00 QuickbuildTrinity Desktop Environment

The result was that I was not allowed to download and then install
some TDE packages.  How to fix?

Regards, Ken Heard