SPF
SPF lists the servers or providers allowed to send mail for a domain. It helps receiving systems compare the sending source with the published policy.
DKIM
DKIM uses a public DNS record and a signature added to messages. It helps show that a message was signed by a domain-controlled key and was not changed in transit.
DMARC
DMARC tells receivers how to handle mail when SPF or DKIM alignment fails and where to send aggregate reports.
FAQ
Do I need all three?
Most serious domain setups use all three together.
Should I start DMARC at reject?
Usually no. Many domains start with p=none while they review legitimate senders.