Negative space is the area which surrounds the main subject in your photo (the main subject is known as the “positive space”). This definition is rather abstract, so take the following example; 

Photo by Robin Mehdee

Negative space defines and emphasizes the main subject of a photo, drawing your eye to it. It provides “breathing room”, giving your eyes somewhere to rest and preventing your image from appearing too cluttered with “stuff”. All of this adds up to a more engaging composition.

Insect on a stem offset by negative space
Photo by Ruben Alexander