DMARC builds on SPF and DKIM. It's a TXT record at _dmarc.yourdomain.com that says what receivers should do with mail claiming to be from your domain that fails authentication: nothing (p=none), send it to spam (p=quarantine) or block it (p=reject).
It can also ask receivers to send you reports, through the rua tag, so you can see who is sending mail as your domain and whether it passes.
A new outreach domain usually starts at p=none with reports turned on, then moves to quarantine once legitimate mail is passing.
Example
v=DMARC1; p=quarantine; rua=mailto:dmarc@margin.example on _dmarc.getmargin.example sends failing mail to spam and reports to Margin's main domain.
Putting it into practice
Where to use it: SPF and DMARC record generator, SPF, DKIM and DMARC checker and Email deliverability for link outreach.