Negative space is the space around a subject. If there are multiple subjects, it is also the space between them. Sometimes a nice image calls out to you for a little negative space, such as this image of Stella Seet:
Stella photographed against a somewhat narrow wall.
Adding negative space to on the left side would give the image a more graphical appearance, as well as unbalance it a little:
Negative space added to the left.
Although adding negative space is a great idea to add graphical interest, it cannot be used in all photos. The space must be clean and simple. Otherwise, the image becomes too busy and the subject gets overwhelmed by the background.