bonus footage; extra video content
Refers to supplementary video material included with a media release, such as behind-the-scenes clips, interviews, or making-of documentaries.
This DVD includes a lot of bonus footage.
You can watch the director's interview in the bonus footage.
特典 is a general term for a bonus or privilege, while 特典映像 specifically refers to bonus video content.
映像特典 is a less common variant with the same meaning; 特典映像 is the standard order.
Compound of 特典 (bonus, privilege) and 映像 (video, footage).