See diagrams 1 and 2. You have a light source and an obstruction which is opaque.
On a screen a distance, you find the shadow. If the light rays travel in straight lines, the regions A, B and C all should be totally dark. The remaining region on the screen is illuminated.
Due to the phenomenon of diffraction of light, the regions B and C are also illuminated but with less intensity.
This phenomenon of bending of light around obstacles or corners and edges is called diffraction.
Due to diffraction of light, different light rays with different path lengths fall at the same points. Then interference of the two light waves takes place. If they combine constructively then constructive interference happens. If the two light rays combine destructively then intensity of light at that point is reduced.
On the screen you will find regions of darkness and brightness alternately. This is called the diffraction pattern. These regions are of equal width.