What does harvest now, decrypt later mean?
It means an attacker records encrypted traffic today and stores it, to decrypt it once a capable quantum computer exists. Recording is cheap and invisible to you: it can happen at an internet exchange, on a compromised router or at a provider, far from your own network. Only traffic whose key exchange is quantum-vulnerable is at risk, and only data that will still matter when it is decrypted. Health records, designs, source code, long-term contracts, legal files and personal data kept for many years are typical examples; this week's delivery schedule is not. A common way to weigh it is Mosca's inequality: if the years the data must stay secret plus the years your migration takes are more than the years until such a computer arrives, the time to start is now. The practical response is to move the key exchange that protects long-lived data to hybrid post-quantum methods first, because that is where today's recordings would become tomorrow's leaks. Signatures can follow later.
This answer is part of the guide: Post-quantum readiness: where to start