A storyboard is a sequence of drawings. It shows the flow of the action and the animator's intentions for the project they are creating. Usually you should add text describing what is happening and arrows that show in which direction things are moving. It should, however, be clear enough in the drawings what is happening without descriptions. Storyboards don't have to be a work of art, they can be simple, but they have to clearly show what you are going to animate.
Storyboard Overview
Tips for making a storyboard in Photoshop
Tips for cleaning up a scanned storyboard in Photoshop