You're a fugitive on the loose. In order to hide from the cops' floodlight, you run through a door and stand against the wall a few feet away from the door. The light doesn't touch you. Why? Because light doesn't diffract.
Diffraction is defined as the bending of a wave around an obstacle or through an opening. To the naked eye, one can only see no diffraction. As explained in the fugitive example above, light does not diffract. Sound, however, does. Therefore, as the fugitive standing where you were, you can hear the police sargent asking you to come out, even though her flashlight doesn't touch you.
Diffraction occurs only when there is a large wavelength and a small obstacle. Likewise, diffraction does not occur when there is a small wavelength and a large obstacle. When a wave passes through an opening, and it diffracts, the wave fronts will bend; when it doesn't diffract, the wave won't bend.
Diffraction is easily testable. Put your forearm in front of your mouth. Now say something. Did the other person hear it? They should. Why? Because the sound waves bend around your arm as they diffract. Now put your hand over the front of a flashlight. Some light may shine through, but is any light coming out around your hand? No. Why? Because light doesn't diffract.