Your nostr key, wrapped by a passkey. This page holds nothing: the backup is ciphertext on a public relay, and only your passkey (Face ID / Touch ID / hardware key) can open it.
Honest limits: lose every device AND every passkey and the backup is gone forever. Add a second passkey (run the backup again with the same nsec and a new label) for redundancy. The unwrap hands your nsec to this page briefly; paste it into your client, then close the tab.