Negative space is simply the space around the subject. With negative space you define and emphasize the subject in the photo. It draws your eye towards it. It gives ‘breathing space’, by giving your eyes the space to rest somewhere, without the image immediately becoming restless or too busy. These things together ensure a better composition. When negative space is used in a good way, it provides a natural balance with respect to the positive space. To get this balance right is more difficult and also very subjective. In any case, it is certainly something that makes you better when you do it more often.