In marketing, a mockup is typically used for promotional purposes. Mockups allow you to visualize a project. It is a great tool to present in a marketing pitch because it allows you to use visuals to describe your ideas. 

There are many forms of a mockup:

  • Print
  • Device
  • Branding
  • Packaging
  • Product
  • Social Media

Source

Here is an example of a website mockup. It shares what will go where and gives you a general idea of what it will look like.