I found an interesting comment recently: "It is the complexity of melody which makes music beautiful, just as negative spaces make a painting work. When next at the easel, remember we are making music for the eyes". Mary Kilbreath, a wonderful artist who paints in oils, made this remark. When you look at her paintings, she does indeed use negative space wonderfully.
Negative space in art is the empty space between delineated objects, the area where the eye can rest. It allows a very strong underlying composition to be woven into a drawing or painting, directing the eye around the art in subtle fashion. It allows a rhythm between the positives and negatives, similar to what Mary Kilbreath was saying about music, the pauses and silences highlighting the melody.
The classic demonstrations of negative space usually use pedestalled urns which also read as silhouetted faces, but I found this Moth (courtesy of Donald Mackay) was an interesting way of showing positive and negative in simple fashion.