Pixel density is the specification that defines how detailed a display is. It is expressed in units of DPI (Dots Per Inch) or PPI (Pixels Per Inch). DPI is typically used for OS and applications, and PPI is typically used for LCD panel specs. For example 96dpi means that 96 dots are placed in a 1 inch (2.54 cm) length. The level of detail that you can see on the screen depends on the distance it is viewed from (e.g. viewing a monitor versus a smartphone), so whether or not a certain dpi is high resolution or not depends on the device. However in general the larger the dpi value, the higher the definition.
Display resolution of a digital television, computer monitor, tablet, smartphone or display device is the number of distinct pixels in each dimension that can be displayed.