Negative space is a very positive means for relating different parts of a drawing. The best known example for demonstrating this principle is the vase/face illustration [animation shown below]. There is every chance you will have seen this somewhere before. In this example the eye is capable of discerning two faces or a vase, but not both simultaneously. When drawing, it becomes a very useful device for line placement. As an example try sketching your hand simply by following the contours of the fingers with your eye. Try the same sketch, but now observe the ‘negative’ space – i.e. the gap between those fingers [see animation bottom-right]. With practice it becomes fairly easy to jump between positive and negative space to better relate curves and lines to one another, and more accurately capture those all important angles that can so often distort and kill the realism of a drawing or sketch. By using the abstractness of negative space, the brain is less likely to interpret an object as it thinks it should be and the all important right-side of the brain can take the helm.
The principle is very simple, and so there is not a great deal to discuss, but the practice of seeing negative space is absolutely crucial, and every time you come to draw, look at ways in which you can utilise it, be it the gap between your pet dog's legs, the space between branches of a tree, or the area that defines the profile of a face (a "half" vase).
Negative Space - Deer's Legs Example
If you wanted to draw this deer's legs, then the best approach would be not to focus on the legs, but the negative space between them (marked in blue). This abstract blue shape is easier for the brain to process as it only knows it as a strange non-specific shape and not as the conception of a deer's legs. These are the sorts of habits and principles you need to teach yourself when seeing, and it will go a long way towards improving the accuracy of your basic drawing skills.