Negative Space is the technique of using elements of low impact to contrast your subject in a picture. Negative Space isn’t always a flat color, its a contrast to the high impact nature of the subject. This could be a flat color, but could also be an area of low contrast, low texture, subtle gradient, etc.
This concept in making images is similar to how we exist as human beings. If we don’t get a balance of space and activity we can feel claustrophobic. This concept works in visual composition in much the same way. Having a balance of high and low impact activity can emphasize the subject in the photograph as well as give a natural and calming desire to the viewer.