ARC lets a trusted intermediary (a mailing list, a forwarding service, a security gateway) attach a signed record of what SPF/DKIM/DMARC evaluated to before it modified or forwarded the message. The final receiver can then consult that chain instead of only its own, post-modification evaluation.
This matters because forwarding and mailing-list processing routinely break SPF (different sending IP) and sometimes DKIM (body or header rewrites), which would otherwise fail DMARC even for genuinely legitimate mail. ARC gives receivers a documented reason to trust it anyway.
ARC is a signal receivers may use at their discretion, not an override of DMARC policy; a domain enforcing p=reject can still see forwarded mail rejected if the receiving side doesn't honor the ARC chain.