The video element can be used to add videos to your publication. Maybe the video you found is from Youtube or Vimeo, or maybe it is your own video that you have uploaded to youtube or vimeo, then you simply take the video URL and use this to implement it into your publication.
To add a video to your publication, simply click the add button and go to the images section, then click and drag the video content block into your publication. Then in the configure tab you simply paste in your youtube or vimeo video link and configure its settings.
Be sure to also read about the browser policies on autoplay videos.