That's a great article by Robert C. Martin. It's one of those articles that really may change something in your programming. The principle is called Open-Closed and it says:SOFTWARE ENTITIES (CLASSES, MODULES, FUNCTIONS, ETC.) SHOULD BE OPEN FOR EXTENSION, BUT CLOSED FOR MODIFICATION.More about that here.
If you are wonder what exactly "Retina display" means I'll suggest to check this great article in SmashingMagazine. I searched a lot for a simple explanation and that's the best one.
