According to TechCrunch, the Watch Later button appears in the upper-right-hand corner of videos when users who are part of the test pass over that area with their mice or scroll past it.
Users who click the button can save videos that appear in their News Feeds for viewing at a later time.
Facebook quietly added videos to the types of files that can be saved last December, but the Watch Later button is much more user-friendly.
Readers: Are you seeing the Watch Later button on videos in your News Feeds?
Screenshots courtesy of TechCrunch.