PGP is a tool for secure communication and data storage, especially for use with e-mail. It can be used to encrypt data and to create digital signatures. IMP uses GnuPG (GNU Privacy Guard) to provide this functionality.
If this option is selected, you will be able to create/view your personal PGP keys, add/delete public keys from other individuals, read encrypted mail, verify digitally signed mail, and encrypt/sign your outgoing mail.