Tone is the relative lightness or darkness in an image. The more variety of tones you have within an image, i.e. the more shades of grey, the better the image. This applies especially to monochromatic images.
Mist is a great tone creator. Subjects closer to the camera will be darker and subjects far away will be lighter.
Ansel Adams created a zone system, where every image needs to have as many shades of grey (zones) as possible. His system included 10 zones, so he aimed to have 10 shades of grey in every image he took. He was the man, so copy him.
How you can take a similar image