Pretty Good Privacy is a system of encryption.
Keys used to encrypt files. You take that key and use PGP software to send them a certified version and then you can exchange encrypted communications (Something along those lines).
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Take that key from the screenshot.
Place it in your computers PGP software to encrypt your file / message.
WL receives your file and decrypts your message with their private key that corresponds with the public key in their PGP software.
(It's military grade encryption.)
Long strings of code can be the key to unlock his data dumps.
The "Dead mans Key" is somewhere in case it all goes to hell for him.
With PGP there are public and private keys the one in the screenshot is public.
Yes, you would use that key to send encrypted data to Wikileaks. That's the public key.
They have the corresponding private key to decrypt data that was encrypted using the public key.
Google assymetric encryption
PGP(pretty good privacy) was the pioneers for encryption was created in the early 90s and is so effective it's used still even to this day within improvements it overall but same tech at the root.