"Retina" displays are becoming more popular. Apple has invented this term "retina displays", and has featured this in their product line (iPhones, iPads and iMacs) from the days of the iPhone 4 (2010). These displays have a higher resolution, about 4x as much pixels per square unit. The result is a much more crisp screen display, more detailed and fluent.
Other display manifacturers have started delivering also high-resolution displays, and it can be expected that this will only increase in the near future. Important terefor to be prepared for this with your webshop, to offer your visitors a perfect shopping experience with high-res photos and other "visuals" that distinguishes your webshop from the rest. You can make sure to use beautifull, big product images; we made sure our theme "Berlin" is ready for these high-res displays. By ensuring that as many graphics as possible render fine on retina dispalys (such as replacing icons with other technics like webfonts, and to make use of separate "retina" versions for other graphical elements).
"Berlin" offers the opportunity to upload your logo and cart icon for regular screens, and separately for high-res displays. For "retina" images you need to upload a separate image with twice the pixel dimension. So if your logo is 300 x 200 (wxh) pixels on a regular screen, then the "retina"-version logo should measure 600 x 400 (wxh) pixels. The retina technic scales this double-sized image back to the right size.