YouTube watermarks are the small icon that you’ll see on the bottom right of many YouTube videos. In reference to these watermarks, YouTube has stated that “…you can use a branding watermark to embed your channel logo across all videos on your channel. When you add a watermark, viewers can directly subscribe to your channel if they hover over the watermark when using YouTube on a computer. It won’t show to users who are already subscribed to your channel.”
The watermark is only available on computers in landscape view and mobile devices. Also, keep in mind that watermarks are not clickable on mobile devices at the present time.
YouTube suggests that watermarks have a transparent background to make them less distracting for the viewer.