Open-ness in hardware terms can have a whole range of meanings. It sounds crazy that people could sell hardware and not tell you how to use it, but it is actually a growing trend. The design of the hardware must be available.
Designs can be free in exactly the same senses as software can be free.
If the design is published others can learn from it, improve on it, or even second-source the same device. Much digital hardware has never been open in this sense, creating a whole industry of reverse-engineering and patent claims and counter claims. Here the lessons from open-source software as a tool for companies with a minority market share have been successfully imported to hardware. Although many designs have always been patented, there is also a long tradition of engineers borrowing and improving on designs that just 'float around'. Design software for hardware must be available.