Domain Message Authentication Reporting and Conformance (DMARC) is a mechanism to tell mailbox providers how to handle emails claiming to be from your domain which are not coming from SPF designated sources or signed by DKIM.
DMARC uses a special DNS record in the domain used in the header from. It requires that both the SPF and DKIM are alligned, meaning that the SPF enevelope from domain and the DKIM signing domain must match the one used in the header from address.
DMARC is intended as an anti-fraud measure and its influence on deliverability is minimal.
Incorrectly setting up DMARC will cause loss of email so if you need help, get in touch.