When you create a list you have to choose between two types:

Double opt-in: After sign up the subscriber will receive a confirmation email to confirm his subscription before landing to your list.

Single opt-in: After sign up the subscriber will land directly to your list.

Note:

If you create forms that are linked to your double optin list, so make sure to customize "optin" message within your form and keep in mind that the subscriber should confirm before been added to the list.