Mailing recipients are defined by their email addresses. Multiple persons with the same email address are considered the same mailing recipient. Mailing recipients can consent to receiving mailings from you at their email addresses, then they are active. Or they can revoke their consent, then they are inactive.
Recipient email addresses can also be subscribed to mailing topics.
The Recipients Email Addresses page lets you search for email addresses, optionally using filters to narrow down the results:

