Pixels per inch is how many pixels wide an inch is on your screen. Make sure you are not in full screen and your browser zoom is 100%. Then take a ruler and measure the line in inches. Change the fraction part to a decimal. Then enter the length. Be accurate! The variable will tell you your screen's pixels per inch, or ppi. For example, my screen is 100 ppi.
For more on pixel density, visit http://en.wikipedia.org/wiki/Pixel_density
