When light is passed through an aperture small enough, what you get is not a bright image of a dot, but a bright disc surrounded with more circular rings alternately bright and dark. The bright central dot is called an Airy disk and the pattern that surrounds it is called an Airy pattern, named after George Biddell Airy. This phenomenon is in accordance with the wave theories of light. You may have observed this before but through a rectangular slit and not though a circular aperture but the two have the same concept.
Airy disks are very important because the eye together with many optical instruments have circular openings. The study and characterization of airy discs played a significant role in the assessment of the resolving power of optical devices such as telescopes, cameras and optical microscopes. It is a very important concept when focusing laser light and in the understanding of human sight. This phenomenon also sets a physical limit to the maximum resolution any aperture size can attain. Even with perfectly formed lenses, resolution is still limited by this form of diffraction. In cases when the circular diffractions are bigger than the image itself, the imaging device or process is called diffraction-limited. Airy discs cannot be avoided even with perfect instruments.