Charles wants to encrypt his email communications. How can he do that? Leo says that your email can go through dozens of servers before getting to the person you addressed it to. And everyone along the way can read it unless it's encrypted. It's more like a postcard, but without federal privacy protection. PGP (Pretty Good Protection) uses public-key cryptography, which has two different keys. One public, and one private. Only you can encrypt with the private key, while the public key is used to verify and open the email. You can give the public key to anyone.
Chris doesn't like the Mac Mail app, and PGP tends to screw things up. Leo has been making PGP keys since 2005, but when he made them, he didn't keep track of the revocation password or get a revocation certificate so he can't revoke old keys. It's always best to use the most recent key. The way to verify that encrypted email with Leo works is by going to his website at leolaporte.com and checking the PGP link, which is his most current key. PGP doesn't work with the updated macOS Sierra Mail app, though.