This is a really cool transition designed by Eric Johnson where you see the transition from a simple play icon to the background video. The play button has the video in the background and when you click on it, the video from the background zooms in and starts playing.
If you are having trouble with the pen, try the archived copy on GitHub
Play Icon to Video Transition Snippet
See the Pen Aperture video play button/reveal — clip-path approach by Eric Johnson (@ericdjohnson) on CodePen.0