...but this image would be really boring if I weren't.
Negative space is the space around and between the main subject of an image. Negative space adds impact. Some say negative space gives your eye a place to rest although I've never understood that need. Negative space is the place where things are not.
As with much that I do, I usually take my negative space to the extreme, like so...
But like with most things, subtlety works too.