At least where I work, Rapid Prototyping is non functional mocks… be it drawings on paper or low Fi digital tools. You get it front of customers/users quickly and validate your ideas before spending dev time on functional interfaces. Also, the benefit of Low Fi is that you can modify them ad-hoc while they’re in front of you as well as not getting hung up on design aspects (white space, fonts, Loren Ipsum filler content, colors, etc). Check out Jeff Patton’s Story Mapping books. We based our rapid prototyping on his ideas around customer discovery.

'rapid' is about how fast you can react, not how fast you can crank out UI.