RWD stands for resposnive web design. Responsive web design is an idea used in web design to make pages fit to any screen size, whether it's a phone, tablet, or computer. It also works to think of design aspects for each screen size ratio. One design concept may work better on a phone, and another on a computer and responsive web design works with all those designs to make them work on all devices.
Responsive web design makes pages look and be more accessible to users, especially since phones are used more than ever. It also allows for only one website to be used accross all platforms which is easier for develepers to manage. So all in all it looks better across all media devices and with responsive web design it is easier to manage one site rather than multiple.
The process is to start with the smallest version and build the site from there. From here one can do the math at where certain aspects might look the best and apply that px amount to the stylesheet