A RWD allows creating a Responsive Web Applications: basically, the challenge is to be able to arrange the layout in a way that all the information is presented in a readable way, obtaining an optimal viewing experience across different devices, and screen sizes.
In order to achieve this, a RWD generally works with various techniques such as Fluid grids, media queries, and flexible images.
GeneXus includes editing features in order to allow doing a RWD. See GeneXus Responsive Web Applications based on Bootstrap