Negative space is a big empty space, usually white or black (although it may be a different color), which is inside the picture. In general this on the side of the picture and can transmit loneliness and isolation. This is contrary to fill the frame.
Negative space also gives visual impact to the photo and therefore is a good resource to use. Like all resources, you can use all the time because all you do is in vain that all pictures are equal. But it is good to have in mind for variety in the images.
In this picture you can see a classic black negative space. The whole right side is empty and black. This gives the feeling of isolation and loneliness.
The fact that the face is not centered model also helps make the picture more dynamic and less static.
This photo was made in the studio and I leave that empty space to the left just to give the feeling of loneliness. Even the position and attitude of the model (though not see the entire face) indicate that feeling.