ARC is designed to address the security issues related to emails that pass through multiple intermediate mail servers, which can alter or remove existing authentication headers. It allows senders to sign the emails at each step of the transmission, which allows recipients to verify the authenticity of the email even if it has been sent through multiple intermediate servers. It also allows signing multiple times and maintaining the chain of custody.
In summary, the Authenticated Received Chain (ARC) protocol is a method for improving email authenticity by adding a digital signature from each mail server that handles the email, and to allow verifiers to check each step of the chain to confirm the authenticity of the email.