An aerial perspective occurs in vision and is when objects at a distance are blurred, less detailed, and lighter in color than when they are nearby. Aerial perspective is a monocular cue which is used for depth perception, which is used to judge how far away objects are. Monocular cues are named because they can occur only using one eye (as opposed to binocular cues which only occur with the use of both eyes).
When looking at an object in your hand you can discern detailed information about the object and it appears clear. When the same object is moved to a distance it appears fuzzy and blurry. You are unable to discern the fine details about the object and the colors may appear different. This is aerial perspective and it is a way for our vision systems to convey how distant and object is. Think about this the next time you are on an airplane. The landscape below you is less detailed than if you were a few feet above it ground and trees.