Negative space is a effect used to strengthen elements in the frame. You don’t need to use just one colour, but the image above is a typical example where the lack of details in the mist actually draws attention to the two trees.
This effect can be used to emphasise leading lines in an image or to draw extra attention to subjects/objects that have been framed.
During the winter the snow can give you some extra opportunities to frame these negative spaces. Other typical elements can be silent surfaces like a long exposed sea / calm sea or uniform patterns like grass or tiles.
The use of negative space can be quite difficult, but it is a very interesting element to add into your frame. Why don’t you go out in your close surroundings and try it out?