Diffraction is a topic that you can make as simple or complicated as you wish. I will try to stick to the basics. Diffraction is the apparent bending of light around an object. Diffraction is an extremely important topic in macro photography as it is often the major determinant of image resolution.
Diffraction doesn't really bend the light as it goes around an object. A good analogy to think about is a wave coming through a gap in a seawall. The wave will hit the gap and it doesn't just continue in a straight line, the wave will radiate out in all directions from the gap in a circular pattern. Light acts in a very similar way.
Diffraction in a lens is most prominent at the aperture and the size of the aperture determines the amount of diffraction. When the aperture is small, diffraction is increased. When the aperture is large, diffraction is decreased. As light traverses the aperture all of the waves that get through will start to interact with each other. If the crest of a wave meets another crest, they will add together to form a larger wave. Two troughs will add to make a bigger trough. A crest and a trough will cancel each other out.
All of this interference is happening on a dizzying scale on the way to the camera detector. The pattern that light forms is a well-defined pattern called a diffraction pattern. The pattern is a series of light and dark bands related to the interference of the waves. The center band of light in the pattern is the most important and contains most of the light energy. The width of this center band of light is related to the size of the aperture. Small aperture makes the band wider (more diffraction) and a large aperture makes the band more narrow (less diffraction).
This center band of light is termed the Airy disc. The Airy disc is the smallest size that a point of light can be focused on the detector and is a direct determinant of image resolution.
