Pretty Good Privacy (PGP) is an encryption program that provides cryptographic privacy and authentication for data communication (Wikipedia).
To use roundcube for this purpose you have to trust the server on which the keys are stored.
We take into account the following scenario: Alice and Bob want to send encrypted emails.
Alice and Bob create their key pairs by clicking on Settings => PGP Keys => Create.
2) Alice and Bob have to export their public keys.
3) Alice and Bob import their keys: Settings => PGP Keys => Import
4) After the import Alice can see Bob's public key and viceversa
5) Alice sends an email to Bob
6) Bob reads Alice's email crypted and signed