What does the "Unsubscribe Reason" mean in my Contacts export?

Bounce - A permanent failure to deliver the email, based on conditions with the recipient mail server. 

Unsubscribed - Opted out via link in newsletter footer.

API - Unsubscribe was done manually via the dashboard or through other integrated platform.
 
Dropped - Similar to a bounce, if our email service provider has attempted a send to this email before and repeatedly seen bounces, invalid email or spam reports returned, it will "drop" it in order to protect the sender reputation.