Responsive images are the set of images your server can make available for different screen formats. For your images to look sharp on Retina displays, you need to provide an image, double the width it will be displayed at. This can be quite large if you want to cater for large desktop screens. That same image on a small mobile screen will be far larger than needed. If the user has a slow internet connection or limited data, you end up degrading the user experience with a far larger image than needed. By making smaller images available and adding some markup to your images, you can let the user browser know what image sizes are available and let it choose the best one for its own display size.