Storyboards, also known as Scenario, are used to prototype an experience, they focus on the tasks, how they are related to each other without thinking much about the details, the technical aspects or the aesthetics. They are usualy used in Service design.
The idea is to draw quickly (with simple shapes, to not divert testers' attention with nice, artistic visuals) the experience, step by step by including the environment, the people involved, the motivations... everything that is part of the whole experience.
You can show the storyboard to users and see if they understand the concept, if they feel like they could be one of the person of the storyboard, if they have any question regarding the steps...
More generally, Storyboards are a good way to visualize the sequence in which the users will use your service or product and can help see everything as a whole.