A mock-up is a smaller model or replica of something that will eventually be built or that's used for teaching. When an engineer designs a new kind of airplane, she'll first make a mock-up.
A mock-up can be a prototype, or a first version of something that will be improved upon before the final version is constructed. Another kind of mock-up is a page of a newspaper or magazine that's laid out so it can be rearranged and perfected before the finished publication is printed. Some mock-ups are full size, but made of cheaper materials than the final product will use — a furniture maker might use this method.