Prototyping is a method of Theory U. Once the seeds of something new have solidified (crystallized), the project enters the prototyping phase. 

This means that you can move quickly into action, learn by doing, and learn from your mistakes. The team will make mistakes and learn from them together. The feedback you get leads you to the next step in your journey. 

That means you don’t have to wait until you have everything together, you can start right away with what you have. Prototyping is trying things out, experimenting, trying to put your feelings/ideas into a manageable form.