The teaser element should “tease” to another source such as a news article.
If you want to open a video when the teaser is clicked instead of a page, tick this option, and enter in the video id. Do not put the video url into the URL section.
This will open a video in a modal, which looks like this:
Here is an example of a teaser element with a link to a video.
Be aware of the size of the picture you choose as it will change the size of your teaser block.