If a subscriber is marked as "Unsubscribed" but their email address exists in a data extension to send an email to, will they be filtered out and not be sent an email?
It depends on your send classification. An unsubscribed status will prevent someone from receiving anything classified as Commercial (or marketing) email. For Transactional email, unless you specify otherwise, an unsubscribed email address will continue to receive mail.
You would want transactional messages (like order confirmations, information requests) to still go through even though someone had unsubscribed from Commercial mail.