DomainKeys Identified Mail, or DKIM, is one of three key standards and policies enabling email authentication. DKIM uses public key cryptography to authenticate individual email messages. In other words, DKIM allows receiving mail servers to verify who sent the message. DKIM combined with SPF and DMARC is the best defense against email fraud.
Read more about DKIM and its role in email authentication in "What is DKIM?", part of our "What is Email Authentication?" series.