This is mainly used to reduce the unauthorized use or copyof your files, photos, as well as videos. AnyShare watermark contains two types: username watermark and custom watermark, which you may add on Security->DLP Policy->Watermark Policy.
Username Watermark: When added, files including Word, Excel, PPT, PDF, txt, picture and video, shall be set with Username Watermark before download or preview.
Custom Watermark: When added, files including Word, Excel, PPT, PDF, txt, picture and video shall be watermarked with the custom content before download or preview.
You can also add watermark for the document library. Click Add and choose the library you want to add watermark.