A message passes DMARC when SPF or DKIM passes and the checked domain matches the domain in the From header (alignment). That link is exactly what SPF and DKIM lack on their own.
The record at _dmarc.example.com holds the policy: p=none (monitor only), p=quarantine (spam folder) or p=reject (refuse). With rua= you receive daily XML reports from large mailbox providers showing which servers send in your name.
Sensible path: start with p=none, read the reports, fix missing senders, then move step by step to quarantine and reject. Jumping straight to reject often locks out your own invoicing or newsletter software.
Example
_dmarc.example.com. IN TXT "v=DMARC1; p=quarantine; rua=mailto:dmarc@example.com"