Active contacts are contacts you can potentially send emails to.
For a contact to be considered "active" they need to meet three criteria:
Consent to receive emails from you
Either express on implied consent. Consent status is managed self-serve by contacts on the email preferences page.
Valid email address
Email address validity is managed automatically as part of email bounce management.
Email verified via double opt-in.
This only applies if you're using double opt-in for form submissions. In this case a contact also needs to have a verified email address via double opt-in for it to be considered active.
Active contacts versus subscribers
The difference between all subscribed contacts and active subscribed contacts is anyone whose email address is invalid or their consent status is revoked, expired, not provided or spam reported. Contacts aren't removed from subscription lists when they revoke consent or their email address bounces.
View active contacts
You can view the number of active contacts in your account from the database overview page.
Billing
The number of active contacts in your account is used to determine contact count for monthly billing.