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.
All subscribers versus active subscribers
On the list of subscriptions page you can see how many contacts in total are part of each subscription and how many of these are active. Active subscribers have a valid email address (learn more about email bounce management) and have either express or implied consent (learn more about consent statuses).
When contacts unsubscribe or their email address becomes invalid due to too many bounces they remain on subscription lists but they are no longer considered active.
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.