With Fluttermail you can create both double opt-in and single opt-in forms.
A double opt-in form will mean that a confirmation email is sent to each new subscriber. The subscriber then needs to confirm that they wish to be subscribed to your list (by clicking on the link in the email that is sent to them) before they become 'Active' and receive further emails. If they don't click on the link, they remain 'Unconfirmed'.
The following screenshot shows where to enable or disable this confirmation email. If it is enabled then your form will be double opt-in.