A mockup is an accurate static design of a final product.
A mockup shouldn’t just outline the layout but also present the content and functionality.
Unlike Wireframes, which just gives an idea of the layout, a mockup should look like the final product. A mockup is, like the Wireframe, just a static presentation, this means it is not interactive and clickable like a Prototype.
You can use a pen and paper and draw your ideas.
But to create some more realistic mockups, you need mockup tools.