Responsive Design is a special technique in web-design/CSS that requires Flexbox to work.
With Responsive UIs the UI automatically and intelligently resizes and changes its own layout to better adapt to the size and shape of the current device/screen. This is not possible with most layout systems, but the dynamic nature of Flexbox makes it possible.
A worked example is available on the website and although the screenshots are from an earlier version of FlexBuilder the same process works in the current version today.
Latest info: see Media Queries
These are coming soon. They enable much more advanced Responsive Designs than can be achieved with Flexbox alone.
If you're an intermediate or advanced C# programmer you can easily implement these yourself - just use scripts to selectively enable/disable specific FlexContainers in your UI when the media-query thresholds are reached or exceeded.