The Abstracted LCD Density is measured in dots per inch (see the docs).
The wikipedia article on Pixel density has a helpful section explaining how to calculate this: basically the number of pixels per inch along the diagonal can be calculated by this formula, where x and y are the horizontal and vertical resolution (so that sqrt(x*x + y*y) is the length of the diagonal in pixels), and d is the length of the diagonal in inches:
sqrt(x*x + y*y)/d
So for example, a LG Optimus One P500 is listed as having a screen resolution of 320 by 480, and a diagonal screen size of 3.2 inches, so it would have a density very close to 180dpi.
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