Contacts are all the emails you have entered into for your newsletter since the beginning. If someone has unsubscribed from your newsletter or you have unsubscribed them then they will still be in your contact list, but they will not be in your subscribers list.
Subscribed are the contacts that are currently opted in to being sent your newsletter. Subscribers can be added manually one by one or with an import. Subscribers can also join your list from one of your signup forms or landing pages.
Unsubscribed are the contacts that have previously received your email but have opted out or you have manually unsubscribed.
The number of active subscribers counts to your monthly plan limit, not the total number of contacts.