This is really a pretty simple concept. Simply put it the use of the same or similarly shaped objects in a photograph. Imagine an apple picking basket filled with apples. That is repetition of form. There are countless ways to include this technique into just about any photograph. It can be the center of attention or simply an element within the image, it's your choice. I've read many photography books and none of them ever had pictures to show the 5 fundamentals of composition.
This is a great example for repetition of form. Looking at the box there are three primary repetitions. the vertical strips of sunlight, the vertical strips of wood, and finally the vertical shadows. There are more but those are the primary ones. Also, note the leading line created by the drag mark, it also creates movement.