Diffraction of light waves (or other waves) is the bending of light rays around a very small obstacle when there is an obstruction in their path.
When a monochromatic light source is kept near an aperture (circular) or a slit of the order of 0.1 mm or so.. The distance between them may be quite short such that the waves that reach the obstacle or the aperture are spherical. If the monochromatic light source is far from the obstacle or aperture, then the waves become almost planar at the slit or obstacle.
We keep a screen on the other side of aperture (or obstacle) not far from the aperture. Perhaps less than a meter or so. Now we observe the diffraction pattern of the light waves on the screen.
Each point on the spherical wave in the aperture acts as a secondary light source. The secondary wave fronts so emitted have different optical paths. When the optical path difference is 1/2 of wavelength then destructive interference occurs. So dark fringes (circular) appear on the screen. At the center a bright circle appear due to the constructive interference of light waves. So one can see fringe pattern of equal widths.
Fresnel number = F =
here, a = aperture size
λ = wavelength of the monochromatic light source
D = distance of the screen from the aperture / obstacle.
When F >= 1 , the Fresnel diffraction pattern can be seen on the screen.
If the distance D is very large and incident light waves are parallel wave fronts (distance between the light source and aperture/obstacle is very large), then a different type of diffraction occurs. That is called : Fraunhofer diffraction. In that case, F << 1.