A list-unsubscribe is a non-obligatory custom header addition that presents recipients with a clear unsubscribe link in their mail client. It is either a MailTo or HTTPS, but the former is more common. Popular mail clients like Yahoo, Outlook, Gmail, and the iOS native mail client have the list-unsubscribe feature.
As the name suggests, you will find the list-unsubscribe in the heading of an email in the form of an embedded link. It is included to help recipients opt-out of emails quickly without wasting time looking for the unsubscribe link in the content of the email.
Asides from making unsubscribing easier for recipients, list-unsubscribe also contributes positively to your sender reputation by preventing your emails from being flagged as junk or spam. A damaged sending reputation also affects the email deliverability adversely.