If you've never heard about rapid prototyping it can be defined by the two individual terms. "Rapid" means a speedy action taking place over a short period of time, and a prototype is typically the first model of a project. This process requires fast iterations of design to implement features, test how they work, and fix what doesn't work until you reach a comfortable solution.
This article will go over some of the basics to rapid prototyping and how you can get started. I'll be focusing specifically on website design, but keep in mind that you can rapidly prototype any interface including mobile apps or GUIs for software. I've also included some related posts and popular tools for prototyping.