Warning Banners allow to warn end-users of suspicious emails. In some cases admins choose not to quarantine some emails, for example - low confidence detections. In these cases, the admin can choose to introduce warning banners on the top of the email body that explain the nature of the threat.
Warning banners now include the top detection reasons, written in layman terms. These detection reasons are generated by SmartPhish, explaining why it found this email malicious. The detection reasons help to keep the user more informed, and help to reduce the chance of users falling to the hands the attackers.