What are Active Contacts?
An active contact is defined as a contact that the system believes to be deliverable. This allows you to see which of your contacts are receiving and subscribed to your content. You can find your Active Contacts by going to: Audience -> Contacts
How Active Contacts are Counted
If a contact has been sent an email successfully, and are not undeliverable and/or have not unsubscribed from an email campaign.
Clicking on an unsubscribe link can complete the process that way - A manual "OptIn" field update does not count)
Exporting Active Contacts
On the top of the Contacts page click the 'Export Active Contacts to Excel' Button. Then go to the Exports tab and download your export.

'Export to Excel' will export ALL contacts, including Undeliverable and OptOut