This plugin creates the image files required by the Retina (or any high-DPI) devices and displays them to your visitors accordingly. Your website will look beautiful and sharp on every device. The retina images will be generated for you automatically (you can also do it manually) and served to the visitors. You will be able to control everything from a nice dashboard.
The Pro version adds support for Retina for Full-Size, support for lazy-loading for the responsive images, upload/replace for your Full-Size or your Retina for Full-Size, and various options.
For more information about WP Retina 2x, check the official page about it here: WP Retina 2x.