“Pretty Good Privacy (PGP) is a data encryption and decryption computer program that provides cryptographic privacy and authentication for data communication. PGP is often used for signing, encrypting and decrypting texts, e-mails, files, directories and whole disk partitions to increase the security of e-mail communications.”
PRISM Break does not recommended S/MIME email encryption because of its reliance on third-party certificates from central authorities. Read more here.
OpenKeychain together with K-9 Mail provides end-to-end email encryption. An experimental project is GnuPG for Android by the Guardian Project.
Read the Email Self-Defense guide by the Free Software Foundation to learn how to encrypt your email messages.