The cryptography protecting your data today — keys, HTTPS, signatures — rests on mathematical problems that current computers cannot solve in any reasonable time. A large enough quantum computer could solve some of them. That is what post-quantum cryptography is: algorithms designed to resist both classical and quantum computers.
The threat is not only in the future
You might think: "no quantum computer can break my encryption yet, so I can wait." The problem is an attack called harvest now, decrypt later:
An adversary captures your encrypted traffic today and stores it. Once quantum capability exists years from now, they decrypt it.
If your data must stay secret ten years from now — medical records, trade secrets, personal data — the threat is present, not future. What you encrypt today with classical cryptography could be read tomorrow.
