Definition: A prototype is an early sample, model, or release of a product built to test a concept or process or to act as a thing to be replicated or learned from. —Wikipedia
Why use this tool? If you want to easily create clickable wireframes. A wireframe, is a visual guide that represents the skeletal framework of a website, app or other software that has a User Interface. Wireframing is prototyping techniques that allows you to focus not on the Design, but more on the User Experience.