It means that we have designed the theme layout in a way that it should look sharper on Retina display or High resolution devices. Example is,
– We have used scalable vector icons of Font Awesome instead of image files, so they will look great on Retina display.
What should I do to all of the content looked great on retina displays? Do I need to upload images in the media library in two resolutions?
So it depends on you. You may use high resolution images in Posts to look good on retina display. (* Please take care the image size as well. Don’t use very high image sizes so it may impact on page speed and bandwidth.)
Use recommended image dimensions in order to look great on Retina display. See readme.txt