Diffraction refers to various phenomena which occur when a wave encounters an obstacle. In classical physics, the diffraction phenomenon is described as the apparent bending of waves around small obstacles and the spreading out of waves past small openings. Similar effects occur when light waves travel through a medium with a varying refractive index or a sound wave through one with varying acoustic impedance. Diffraction occurs with all waves, including sound waves, water waves, and electromagnetic waves such as visible light, X-rays and radio waves. 

Diffraction with a small aperture 

Diffraction with a large aperture 

Images Provided by Matlamat Sekolah

What makes it a large or small aperture? It's related to the size of the wavelength.  If the aperture size is the same size or slightly larger then the wavelength then it's considered to be a small aperture.  A large aperture has a size much, much larger.

A simple mix up in the video that needs to be corrected.  Mechanical waves have a medium and a disturbance in that medium is called a wave. But light waves doesn't have a medium and doesn't need to disturb a medium in order to exist (thankful).  

Please keep that in mind