The article is about:

• The Signal Protocol, a set of cryptographic specifications that provides end-to-end encryption for private communications. PQXDH, an upgrade to the Signal Protocol that adds a layer of protection against the threat of a future quantum computer breaking current encryption standards.
• Quantum computing, a new type of computational system that can solve some complex problems faster than classical computers, such as the hidden subgroup problem that underlies many public key cryptosystems4.
• Post-quantum cryptography, a new category of algorithms that resist quantum attacks, and how Signal has chosen CRYSTALS-Kyber as its post-quantum key encapsulation mechanism.
• The implementation and deployment of PQXDH in Signal’s client applications and the open research areas for further quantum resistance.