To mock is to imitate, right? So a mock-up is something constructed to imitate or represent an intended finished product, which could be a series of structures, the façade of one building, a masonry pattern on one part of that building, etc. So, as you can see, a mock-up can refer to several different things.
1) A three-dimensional scaled-down working model that used where the actual objects and/or materials are either unavailable or too expensive to use. It’s intended to represent a design and analyze a design concept. (Frequently called an appearance model.)
2) A full-size model used for display and/or presentation for approval.
3) A full-size model used for testing and reference during work for quality assurance purposes.
4) A working sample (as of a brochure or presentation board) for reviewing layout and format.
See also: façade, masonry