The video block in the email tool allows you to embed YouTube and Vimeo videos into the email.
If the recipient’s email client supports showing videos directly in the email this will be done, otherwise the video will open in YouTube/Vimeo instead. (There are very few email clients who show video directly in the email.)
Now the video’s “thumbnail” is fetched from YouTube/Vimeo. As with images, you should enter an alternative text (a description of the image), and you can resize the video by dragging the slider in the right-hand column. There you can also left / right-adjust or center the video.