what exactly is device pixel ratio?

Question

this is mentioned every article about mobile web, but nowhere I can found an explanation of what exactly does this attribute measure.
Can anyone please elaborate what do

Answer 1

https://developer.mozilla.org/en/CSS/Media_queries#-moz-device-pixel-ratio

-moz-device-pixel-ratio
Gives the number of device pixels per CSS pixel.

this is almost self-explaining. the number describes the ratio of how much "real" pixels (physical pixerls of the screen) are used to display one "virtual" pixel (size set in CSS).