"recipient" of the message in near protocol signMessage function

Question

I'm trying to authenticate account in backend for NEAR protocol

Referring this doc: https://docs.near.org/develop/integrate/backend-login#2-ask-the-user-to-sign-the-challenge

const signature = wallet.signMessage({ message, recipient, nonce: challenge, callbackUrl: <server-auth-url> })

but I haven't understood what to put in recipient and why it is used for