Quantum computing is a tough concept to get your head around ,the simplified version is that quantum computers use the principles of quantum physics, which governs the behavior of subatomic particles, to perform tasks faster than a transistor-based computer. So post-quantum cryptography simply means secure against attacks by a quantum computer. Specifically, Google is using a promising post-quantum algorithm called New Hope. Google is using it on top of the existing crypto algorithm, in case New Hope turns out to be breakable with today’s computers. The post-quantum algorithm protects you from hackers from the future, but it might be vulnerable against today’s machines. Conversely, the elliptic-curve algorithm Google is normally using might be worthless against future’s quantum computers, but it’s the best option against the computers of today.
For years, experts have warned users to use secure browser communications whenever possible. If you visit a site and the URL in your browser’s address bar starts with “https://”, you know the communication between your computer and the server is encrypted. But that encryption, according to Google, has a potential weakness. If quantum computers get sufficiently powerful in the future, they could be used to break TLS, the cryptographic protocol that’s one of the foundations of HTTPS. A future quantum computer might be able to retroactively decrypt today’s internet communications. So Google is addressing the problem today by testing a post-quantum key exchange algorithm in Chrome, which should ideally protect from such future attempts.