What do Magento's various Newsletter Subscriber statuses mean?

Depending on your configuration, you may see four different types of newsletter subscriber status. This is what these statuses mean.

Not Activated

This means that a user has signed up but not yet confirmed their email address. A confirmation email will only be sent if you have turned on the need for a user to confirm their address (Stores > Configuration > Customers > Newsletter > Need to Confirm), otherwise they will be automatically subscribed when they sign up.

Unconfirmed

This means that a user with a customer account has signed up, but not yet confirmed their address for the account creation. This confirmation will only be necessary if you have turned on the need for a user to confirm their address (Stores > Configuration > Customers > Customer Configuration > Require Emails Confirmation). Once they have confirmed their address for the account, they may become "Not Activated" until they also confirm they wish to receive newsletter emails.

Subscribed

This means everything has gone well and the user is subscribed.

Unsubscribed

This means that a user was subscribed to your newsletter, but has since opted out. They should be able to do this both in their account settings and via the newsletter itself.