DKIM is the acronym for DomainKeys Identified Mail. It is an authentication protocol used to validate sending domain names with email messages. DKIM protocol allows email senders to identify the domains that belong to them, thus protecting brand and reputation. This controls spoofing and phishing of your domain name i.e. other senders cannot send emails pretending to be coming from your domain.
DKIM uses cryptographic authentication by inserting a digital signature into the email message header which is later verified by the receiving host to validate the authenticity of the senders domain. The DKIM digital signature is created using a unique string of characters encrypted as the public key and stored in your DNS. When a recipient gets your email signed by DKIM, the public key is retrieved from the DNS Records of the sender and is decrypted to authenticate the senders domain
DKIM Records have certain limitations. DKIM verifies only the senders identity but does not verify the message content. Since email content is also one of the important factors of email delivery, DKIM does not completely guarantee high inbox deliverability.
For more information on DKIM records, you can always visit: http://www.dkim.org/