At the 9 second mark in the clip, you can see Anne Frank leaning out of a second-story window as she watches a bride and groom exit a neighboring address. The Guardian reports that the scene dates back to July 22, 1941 and was provided to the museum by the couple in the 1990s.
The story also goes that in the 1950s, once Anne's diary became public, the couple recognized Anne in their wedding video. So they decided to contact her father, Otto Frank, to whom they gave a 5 second version of the clip.
Annemarie Bekker of the Anne Frank House is quoted as saying:
"The museum has had the footage for some time, but thought YouTube would be a good platform to show the film and the other films about her life. It's another way to bring the life of Anne Frank to the attention of younger people, and all people worldwide."