Transitions occur when moving from one photo to another photo in the timeline. Photo to Movie provides many transitions and options including crossfade, wipe, iris, slide, push, zoom, color fade, and slide changer.
The transitions are represented in the timeline by the horizontal orange line between overlapping areas in successive photos. You can change the duration of transitions in the timeline by dragging the motion paths on either side of the transition.

You can also select one or more transitions(see Selecting transitions) and change the duration(see Setting duration of transitions) or the type(see Transition types) of those transitions all at once.