The video is a comparison of the acting of Eric Stoltz (the original actor who played Marty McFly) and Michael J. Fox (the Marty McFly we know from the classic film).
Robert Zemeckis and Bob Gale had planned for Michael J. Fox to play protagonist "Marty McFly" in the first Back to the Future film. But Fox was already starring in the famous sitcom, "Family Ties". Gary David Goldberg, the producer of the sitcom didn't allow Fox to be casted in "Back to the Future" because it would restrict his time to film on "Family Ties". As a result they got Eric Stoltz (best known for playing Rocky Dennis in the 1985 film Mask) to play Marty.
After filming for several weeks, Zemeckis was unsatisfied with the result of the film, because Eric Stoltz was a method actor and didn't get the humour that was written in the script. They fired Stoltz, and Goldberg allowed Fox to film "Back to the Future". Michael J. Fox worked at "Family Ties" during the day, and worked on "Back to the Future" at night. His only opportunities to film the movie during the day was during weekends. Michael J. Fox worked on average 20 hours per day.
In 2010, for the 25th Anniversary celebration of Back to the Future, a documentary was made releasing previously unreleased footage of Stoltz as Marty McFly, only showing a minimal amount of film. Photos of Eric Stoltz as Marty McFly were released at www.bttf.com