What does this mean? It means that dark colors appear darker than they should. Instead of dim shadow beneath a tree, the shadow could appear as pitch black.
To correct for this problem, the input voltages are mapped according to the monitor's response curve using whats called a gamma correction. The bitmap below allows you to determine the gamma value of your monitor; unfortunately gamma correction needs to be performed by your operating system and many do not.
The gamma value of your monitor is where the bars appear with the same
intensity. Try squinting a bit to blur the patterns. WWW browsers often
remap colors, if the above image is not greyscale, then you'll need to
download it.
Gosh perception is a weird thing!