What is this film about?

This film was created with prejudice and in doing so shows you others' prejudices. If I understand your question correctly, you should view this film as a primary document of the prejudice and bias of Nazi Germany. The version marketed here I think has subtitles. The version which I watched was the Special Edition, which was available at my local movie store. The Sp. Ed. has a fantastic commentary by Anthony Santolini (spelling? my apologies). It is worth watching.

It's almost entirely pageantry and speeches from the Nazi rally. If there is prejudice it's that in the rally the Nazis are proclaiming themselves to be the greatest thing Germany has ever seen. There is no violence. I think Reifenstahl made a great movie showing people that would lead much of the world into war and cause the downfall of Germany.

If I remember rightly there are no references to the Jews or Nazi racial policy.

It is a documentary of the 1934 Nazi Party Congress in Nürnberg. There are no violent scenes. It is just a documentary of a political rally. The rally was significant from a historical perspective because it was the first time Hitler had addressed the rank and file of his party since the Night of the Long Kinves, when Hitler murdered many of the top leaders of the SA, including Ernst Röhm, who was probably the most popular man in the Nazi Party among the rank and file. So the situation was extremely delicate for Hitler. It was very unclear as to how he was going to be received. So the title of the documentary is quite appropriate. Hitler literally won back the support of his party through pure will. Beyond its importance as a historical document, the film is also significant because of the quality of the film itself. Reifensthal to this day is considered the finest female director to ever live and this is one of her finer works.