An early version of a product that has not been manufactured or released. A prototype allows engineers or other testers to test the product before it is manufactured in mass or released to the public. Prototypes are another step that can be taken to help verify no problems exist with the product before it is released to the public or on a larger scale.
To prototype is also a verb, meaning to create a prototype. For example: "Before we develop the software, we should prototype our idea. Prototyping will help us gauge its viability before we commit vital resources to the project."