These are a series of images that communicate a sequence of events in a situation. Much like written stories, the images have a setup, set of related events, climax, and resolution.
Each frame in the story board acts like a keyframe in an animated movie, but the frames in between are left out since they represent user activity. How many keyframes to include in the storyboard depends on how much detail can be figured out by the viewer. A long form doesn’t need a frame for filling out every field, but if certain settings aren’t obvious, it’s good to break them out into separate images.
Actions depicted in the images can and should be annotated to explain what is going on in more detail. Short notes like “user enters their credit card number and expiration date and clicks submit” can clarify what is going on in the image if it’s not completely obvious.
Images can be enhanced with additional visual information. Arrows can indicate something like the swiveling of a person’s head when they notice something, or motion blur can be used to indicate fast movement.