Storyboards are used to

A storyboard is an area where there will be a series of static images or sketches that when played in a fast sequence, will appear as an animation.

If you had lets say 200 static images of a man walking down a street, when its played at 15 frames per second it appears as if the man is now in a movie.

Storyboarding is the traditional way to test out animation sequences but todays 3d software can now replace that.