A Sender Policy Framework (SPF) is an email authentication system that allows the owner of a domain to specify which mail servers they use to send mail. SPF adds a layer of security to your emails by validating that the message is being sent from a legitimate source.
Similarly to DKIM, SPF verifies that Act-On is allowed to send emails on your behalf. If Act-On is not specified in the SPF entry, most email servers assume that we do not have permission to send your emails. SPF authentication does not guarantee inbox deliverability, but without it emails are very likely to be sent to spam or rejected altogether.