What is the “From” header and what does it mean?

The From header is the human readable identity of the sender. You might see something like Captain Kraken from TidalMail Fleet kraken@tidalmail.com. This header has two parts. The display name and the address.

This is the header that DMARC cares about for alignment. The receiving server looks at the domain in this From address and checks whether it matches the domain that was authenticated by SPF or DKIM.

It is the nameplate on the cabin door. It tells you who wrote the message even if a different ship carried it across the sea.