What DMARC does
DMARC tells receiving mail servers what to do with messages that claim to come from your domain but pass neither SPF nor DKIM in a way that lines up with the From address. It is a TXT record at _dmarc.yourdomain that starts with v=DMARC1 and carries a policy: p=none, p=quarantine or p=reject.
It also gives you visibility. With a rua address in the record, receivers send you daily aggregate reports that list who is sending mail in your name.