Double opt-in in context of Nosto pop-ups is a functionality where a subscriber of a newsletter or similar marketing email needs to verify separately that they have subscribed a newsletter. Using double opt-in is provided to support cases where a verification is a legal requirement to include a recipient to an email subscription list. Consult your legal advisory in case you think you are obliged to use double opt-ins for newsletter subscriptions.
In practice the feature works by first configuring double opt-in under pop-up advanced settings. When a user inserts an email address in the pop-up dialogue an email is sent to the given email address asking to verify the subscription. A verification link in the email can also point into your system where double opt-in is done separately or simply works as a confirmation link which will point to your site or landing page where a subscription is confirmed. If the latter mechanism is used, verified email addresses will have value “true” in the named column when exporting email addresses from Nosto.