This is a Symbol library with a set of grid layouts using the Bootstrap framework. The different layouts correspond to min-width and max-width media queries for different viewport dimensions, to help people doing responsive layout wireframes.
Toggling Markup visibility (CTRL+K/CMD+K) hides columns, notes, and top bar to reveal only the square rectangle of the viewport.
To use as a symbol, click the Download BMML link under the Mockup on the page above and save to one of your assets folders to use as a Symbols library, or you can download the BMML file directly from here:
In case you're unfamiliar with Symbols, they are re-usable components that you can customize. You can find documentation and tutorial for using Symbols here. A tutorial for using this symbol library is forthcoming.