Passkeys are a standard pushed by Apple, Google, and Microsoft built on top of webauthn. Passkey essentially solves for the second factor in a two-factor authentication scheme, in order to eliminate the need for passwords. Apple will support things like TouchID, FaceID, and second device.
Passkeys themeselves have no bearing on centralization or decentralization. They are useful for Fission:
Here's a blog post that walks through a potential implementation in a web browser.