What is a storyboard? It's a place to plan out your digital story in two dimensions. The first dimension is time: what happens first, next, and last. The second is of interaction: how does the voiceover (your story) interact with the images, how do visual transitions and effects help tie together the images, how does the voiceover interact with the musical soundtrack?
Here are some worksheets (three different layouts, you choose which you like best) to help you storyboard your digital story.