Physical pixel
Screen size of the unit pixel is a physical device, a pixel in the particles in the different devices can hold 1px are not the same, so this unit there is also a 1px N pixel particles filled. Each pixel in the same pixel particle size of the screen dots can accommodate more display the clearer, the higher the resolution. I.e. an all pixels contained more particles, the better. Now the mobile phone screen with the naked eye could not see the particles, which is due to more and more devices support small particles of pixels, the screen is called the retina (retina screen). Now suppose a 1px picture, the picture receiving one pixel 1px particles, now into the picture screen support 1px accommodates two particles, which particles will split the image into two halves 1 px equipment inside, and because 1px of the device must have two particles to fill up and now it's two particles have not been filled, this will result in display blurred picture. So instead 1px image is displayed on a screen is very clear, then it will certainly be clear on another screen, clearly depends on the physical pixel screen is not clear.
Pixel ratio
Pixel ratio refers to a particle pixel units can be accommodated px
Resolution
= Width * height * resolution than pixels. Figure above 414 * 736 * 3 = 914 112, the screen size can accommodate 914,112 pixels particles. Pc pixel ratio is 1, the 1920 * 1080 resolution is pc * 1080 1920 * 1 = 2073600, 2073600 accommodate the entire pc screen pixels particles.