By entering your birthdate and your expected life span, it projects how far you are into your life by display an image from a movie at that same point as you are at your life today. So if you are 50% through your life, it will pick the 50% point of a movie. Ideally its a movie you know very well otherwise it won't make as much sense.

It's very simple, but not obvious at first.

You enter the age you expect to live, your birthdate, and a movie (pick something you are very familiar with). Upon submission, a single frame of the movie is shown representing today in your life if the runtime of the movie spanned your entire predicted lifespan.