Disables automatically sending emails that have been queued. Instead, each individual email must be either approved or declined within Manage Pending Review.
By default, emails for an email campaign are automatically sent to a customer. The only settings to modify that are Skip If Email Not Sent Within and Send Email After Specific Time which change the expiration and delay time of each email. Beyond that, no other interaction is needed.
Enabling Require Review Before Sending under Advanced Settings allows for each individual email to be placed into a Pending Review queue waiting for your approval. You can preview the actual email that will be sent, along with either approving or declining.
Use of this setting is good for either testing some setting modifications or building confidence with how an email campaign works.