Active subscribers are total recipients that have been uploaded to your account(at the moment) minus those that have unsubscribed, bounced or are duplicates.
Unique subscribers are the number that tells you how many unique email addresses you’ve sent to. It also takes into account bounces, unsubscribes and deleted ones. If you’ve sent to an address – it’s counted as a unique subscriber.
The active subscribers count is being reset each hour. The unique subscribers count is being reset each billing period(monthly). Both these numbers cannot be more than your plan limits, so, for example, if you have a plan for 10 000 subscribers, the Active subscriber count cannot be over 10 000 subscribers and the Unique subscriber count cannot go over the 10 000 subscriber limit.
Can’t find the exact number of your active and unique subscribers? Read this – Where can I find active and unique subscribers?