What Video markup does
It helps Google find your video, understand what it's about and show it with a thumbnail and length. Google requires the name, thumbnail address and upload date, and recommends the description, duration and the video file or player address.
Google shows video thumbnails mainly for pages where the video is the main content, so add this to pages built around the video.
YouTube videos
If you embed a YouTube video, use its embed address (https://www.youtube.com/embed/ followed by the video ID) and YouTube's thumbnail. Google can also find embedded YouTube videos on its own, but markup makes the details clear.