Why do we need to know this? There are two reasons; a prezi looks better if its frames match the format of the output screen. Furthermore, the content of the individual frames presents itself better when it is designed specifically for the frame proportions that you will use when showing the prezi.
Some people think that this part of Prezi is a little hard to grasp. Actually, it's not that bad. Just read on and spend a little time studying the images carefully. You'll understand it all nice and easy.
Oh, and did we mention that it is easy to set the standard format for your frames? It is!