DomainKeys Identified Mail (DKIM) is a system that helps authenticate your emails. Much like SPF, DKIM is an authentication framework that tells the world that it is really you sending a given email message.
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Similarly to SPF, DKIM verifies that Act-On is allowed to send emails on your behalf. If Act-On is not set up with the appropriate DKIM keys, most email servers assume that we do not have permission to send your emails. DKIM authentication does not guarantee inbox deliverability, but without it emails are very likely to be sent to spam or rejected altogether.
To set up DKIM, please see Email From Setup.