Pixel density (Pixels Per Inch) - definition
Pixel density is the measurement of how many pixel can fit in one inch, the measurement unit of pixel density is PPI (Pixel Per Inch). The more the pixel density is, the higher the picture quality becomes. i.e.: iPhone 12 has around 460 ppi on the display panel. It varies depending on the display type and brands and few other parameters. If the pixel density is higher, then the picture quality becomes clearer, the fonts seen smoother and higher in quality.