Retina display is just a marketing term coined by Apple that just refers to a higher pixel density display. It allows the screen to show more detail and improves the viewing experience. But don’t worry if you don’t have an Apple device, many other devices have high pixel density displays; it’s just that they won’t call it Retina.
ShortPixel Image Optimizer can optimize images created for Retina displays. If you enable this option and if you have a Retina plugin that generates Retina-specific images (ending with the suffix @2x), the plugin will optimize them as well.