The record is a TXT record at _dmarc.yourdomain. A message passes DMARC if SPF or DKIM passes and the domain that passed is aligned with the domain in the visible From header. Alignment is the point: it ties the technical checks to the address the recipient actually sees, which is what phishing imitates.
The p= tag sets the policy: none (only monitor), quarantine (treat as suspicious, usually the spam folder) or reject. rua= names the mailbox for aggregate reports, which receivers send daily as XML summaries of who is sending mail with your domain. sp= sets a separate policy for subdomains, and adkim and aspf switch alignment between relaxed and strict.
The usual path is to start at p=none, read the reports until every legitimate sender passes with alignment, then move to quarantine and finally reject. Large mailbox providers now expect bulk senders to have a DMARC record. A policy of reject is an instruction to receivers, not a guarantee: each receiver decides how to apply it.
Example
_dmarc.example.com. 3600 IN TXT "v=DMARC1; p=quarantine; rua=mailto:dmarc@example.com; adkim=s; aspf=r"