Call me old and Dumb or .. just dumb if you prefer but .. I still don’t have a clue.
PGP stands for “Pretty Good Privacy”, a publicly available encryption protocol. The big block of “gibberish” is his public key, which you could use with your own private key to send him encrypted emails.
PGP is “pretty good privacy,” and encryption program that uses so-called “public keys.” Now, it might sound strange that highly secure, secret messages could be sent in public, using keys that are publicly posted, that is exactly what PGP offers. The text string you ask about is his public key.
Using the PGP program, you can read in Sourcery’s key, and encrypt a message with it, which would be all but indecipherable to anyone (possibly except the NSA) except for Sourcery, who could decrypt the message himself. So you can send him something privately, using the key he has posted, and be pretty dang sure no one else can read it except for him and whom else he has disclosed the secret part of his key to.
To go into a little more detail, the public key represents a composite number, which is the product of two large prime numbers. It is extremely difficult to factor the composite number into its prime factors. The public key is the product, one of the prime factors is the secret key that only Sourcery knows. If you “mix up” (encrypt) a message with the product, it is impossible to decipher unless you know the prime factors ... which it isn’t easy to find given the product.
Better?