Responsive web design (RWD) is a design paradigm for web applications that allows to dynamically adapt the user interface to the physical conditions of the end user device.
In other words: A web application renders its user interface differently depending on the viewport of the browser. For example, a web page with a three column layout on a desktop monitor might switch to a two column layout on a tablet and a single column layout on a mobile phone. Or the application switches from a one column layout in portrait orientation to a two column layout in landscape orientation.
Nicely applied RWD patterns also allow to optimize a web page in terms of resources. A low resolution viewport makes the application display low resolution images, whereas on a high resolution screen the same application loads a high resolution version of the embedded images.
For load testing, RWD means that testing an application from more than the one usual end user perspective becomes necessary. Beside different user groups, different browsers, and different network conditions, you also need to consider the end user device or viewport, which adds another dimension in test combinations.