For its work the application uses two types of keys: public and private. The public key is used to encrypt files and can be easily transferred by any available means. The private key is used to decrypt the file, its confidentiality is most critical for the safe exchange of information.
The recipient of the file creates a pair of “private-public” key, sends the public key to the sender. Encrypting and sending the encrypted file to the recipient comes using public key. Decryption is performed by the recipient with help of the private key.
More details can be found on the page of PGP Files.