When crafting the recipe to automatically add subscribers to MailChimp from another system, you might come upon the double opt-in option.
If you select yes, then all the subscribers you are adding via the recipe will receive an email from MailChimp asking them to confirm that they have opted in to join your mailing list. Until they opt in via the email from MailChimp, they will not be added to your mailing list.
If you select no, then all the subscribers you are adding via the recipe will be automatically be added into the specified mailing list in MailChimp.