In plain words
A message can pass SPF or DKIM for some other domain, such as a newsletter tool’s own. For DMARC that pass only counts when the domain matches yours. Relaxed alignment accepts a subdomain of the same domain; strict needs the exact name.
Why it matters to a firm
A service that sends in your name without being set up for your domain fails DMARC, and under a reject policy its mail is refused.
The check behind the finding
Open a message’s headers. Compare the From domain with smtp.mailfrom= and header.d=.