The short version
Almost everything private you do online like logging into your bank, sending a message, entering a password, paying for something is protected by encryption. That encryption works because certain math problems are too hard for today's computers to solve in any reasonable amount of time.
Quantum computers change that assumption. A large enough quantum computer could solve some of those "too hard" problems quickly and break a big chunk of the encryption the internet relies on.
That machine doesn't exist yet. But the threat is already here, for a reason we'll explain. And the world's security agencies are already rolling out replacement encryption designed to survive quantum attacks. This article explains, what the threat is, why it matters today, and what's being done about it.
How encryption protects you right now
