The ratio between whitest white and the blackest black. A greater contrast means the greater the ability of the projector to show subtle color details even in light rooms.
There are two separate measurements of contrast ratio:
Full On/Off contrast is the measurement of light out when the projector displays a all white image (Full on) compared to the light output of a full black image (Full off).The second is the ANSI (American National Standards Institute) contrast; this is measured by projecting 16 alternating black and white rectangles. The average light output from the white rectangles is divided by the average light output of the black rectangles to determine the ANSI contrast ratio.

Be sure you are comparing the same measurement standard when comparing contrast ratio of projectors.