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 give 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.