What does zero-knowledge mean in a password manager?
The short answer
Zero-knowledge means only you can open your vault. Everything is encrypted on your own device with your master password, which the provider never knows. Even if their servers are hacked, or a government comes asking, there is nothing readable to hand over.
In detail
In a zero-knowledge architecture, all encryption and decryption happens on your device. What travels to the provider's servers is already unreadable ciphertext. The key (derived from your master password) never leaves your device. The provider literally cannot look into your vault. Not for support, not for advertising, not under legal pressure.
This is the standard among serious managers. 1Password, Bitwarden, NordPass, Proton Pass, Keeper, and Dashlane all work this way. The differences are in verifiability. Open source managers like Bitwarden and Proton Pass let anyone inspect the code, and periodic external audits confirm the architecture holds. 1Password adds an extra Secret Key, so even a weak master password is not enough for an attacker.
The flip side is in your hands. Because nobody can reset your master password, the safety net (recovery code, emergency contact, or family recovery) is something you set up in advance. Do it on day one.