When a contact subscribes through one of your double opt-in signup forms, that contact is suppressed until they confirm their address. We send an email containing a confirmation link to that contact immediately when they sign up. Before the contact clicks that link, they are suppressed under the label of Unconfirmed. After the contact clicks that link, they are automatically placed into the correct list in your active subscribers.
You can also see these unconfirmed signups in the list called Signups. From there you can select the unconfirmed contacts and manually confirm them.
Please be careful! Only manually confirm signups that you recognize, or know are valid. The point of the double opt-in form is to keep spam bots and other malicious characters OUT of your active subscribers.
And remember, no contacts in your suppressed list ever count towards your active subscribers total.