DKIM (Domain Keys Identified Mail) is an email authentication technique that allows the receiver to check that an email was indeed send and authorized by the owner of that domain. This is done by giving the email a digital signature. This DKIM signature is a header that is added to the message and is secured with encryption.
Once receiver (or receiving system) determines that an email is signed with a valid DKIM signature, it’s certain that parts of the email among which the message body and attachments haven’t been modified. Usually, DKIM signatures are not visible to end-users, the validation is done on a server level.
Implementing the DKIM standard will improve email deliverability and will protect against malicious emails sent on behalf of your domain. Though, in practice these goals are achieved more effective if you use DKIM record together with DMARC (and even SPF). DMARC and DMARC Analyzer use both SPF and DKIM. Together they provide synergy and the best result for email security and deliverability.