A minus value in highlight tone darkens the highlights (and the opposite for a positive value - ie lightens the highlights)
A minus value in shadow tone lightens the shadows (and the opposite for a positive value - ie darkens the shadows)
If you set the camera to shoot RAW, then you can use the in camera raw convertor to experiment with these settings, and create a series a jpegs with different settings applied and chose the one you like the most
