The pixel density indicates the distance between the pixels on a display and is calculated from the image resolution and the display size. Pixel densities of 400-500 ppi (pixels per inch) are not unusual for smartphones and tablets, but they also reach values of over 800 ppi in some models (Sony Xperia XZ Premium).
The pixel density is largely responsible for the subjective impression of the screen's sharpness. Apple markets iPhones and iPads with the advertising term of Retina display. However, this only means displays with a certain pixel density. Retina displays are supposed to have such a high pixel density that individual pixels are no longer visible to the naked eye at a natural viewing distance.