HiDPI refers to a mode in OS X Lion and subsequent versions that forces OS X to render UI elements at double the normal resolution. The mode was disabled until the release of the retina MacBook Pro in the summer of 2012, which has enough pixels to display HiDPI mode correctly
How can I change the resolution of my MacBook Pro 13" Retina (Late 2013) to 1920x1200 pixels? The MacBook is able to use this resolution! It's e.g. using it, when connecting a second display which ...