“Harvest now and encrypt later” means there's no protection on the Internet that prevents all the data packets that go around everywhere around in the world from being intercepted. They can continue on their way, so it's not noticeable, but you can take them and store them. That's what “harvest now” means. You can harvest at the moment all the information that goes on the Internet, even though some of it is protected by cryptographic systems in the current cryptographic infrastructure.
Those that are currently protected maybe—I only say “maybe”, because we don't know. Nobody knows how to decrypt them today without knowing the key. This information that is harvested includes all you'll need to decrypt it later when a quantum computer becomes available.
When there's a full-scale quantum computer, which we, hopefully, don't have yet, but there's no guarantee.... Maybe there is one running in some agency's basement somewhere. In the system, there is no full-scale quantum computer yet today, but there will be one. There is no doubt at the moment that full-scale quantum computers will finally become a reality. At that time, all of this information that has already been harvested can be taken back and decrypted retroactively.
That's what I meant when I said that everything that's been on the Internet will become an open book. There's no point trying to save the past. It's gone forever.