It’s a nip46 signer. Key stays in clave and clients request that clave sign on your behalf. So you can test out any number of Nostr clients without handing over your nsec to each of them.

GitHub

clave/FAQ.md at main · DocNR/clave

Push-based iOS NIP-46 Nostr remote signer. Wakes a Notification Service Extension via APNs to sign events in the background. - DocNR/clave

Clave

Clave — Nostr remote signer for iOS

Sign every Nostr event without exposing your keys. Your nsec stays on your iPhone in the iOS Keychain — device-only, never synced or backed up.