When a video is marked as "preview" on YouTube, it means that the video is not yet available for public viewing. Instead, it is only viewable by a select group of users who have been granted early access. This is often done as a way to create buzz and anticipation for the video's official release. Additionally, creators may use the preview feature to gather feedback and make final adjustments before the video is made available to everyone. Once the preview period is over, the video will be made public for all users to view.