Mock-ups are a set of designs in the form of screens, diagrams, snapshots etc., that helps verify the design and acquire feedback about the application’s requirements and use cases, at an early stage of the design process.
A mockup is a conceptual tool that is used especially in web development. It is basically an early draft of a website or web application. Mockups are primarily used for conception to convert ideas and concepts into a concrete design. They typically include the final navigation structure and detailed design elements so that they often resemble the final design of the website.