In the test card above the numbers indicate the percentage brightness of each band, the leftmost one being totally black and the rightmost one totally white. All the numbers from 5 to 95 should all be visible, though to see the 5 may require room lighting to be subdued. If they are not all visible then you are losing detail in either the shadows or highlights of any images that you view.
To optimize your monitor, adjust the brightness and/or contrast so that the 5 and 95 are just visible. You may have to refer to the instructions for your monitor in order to do this. If it is still not possible to see the full range of numbers, it may be necessary to make further adjustments to the overall brightness level inside the monitor (on older CRT models) and this may require specialist knowledge if it is not covered in the instruction manual.