Brightness controls on your TV may also be called black level - it's used to adjust the darkness of black images. Set the brightness too high and images will have surface whiteness that will make colors look washed out. Set it too low and images seem to descend into murky pools of blackness.
To adjust this setting, find an image on a DVD presented in a widescreen format that has some dark and lighter images on the same screen and pause. The letterbox bars on the top and bottom of the screen are what we'll use to correctly set the blackness but the rest of the image should look natural to your eye.
Letterbox image with both dark and light picture.