Diffraction is when a wave spreads out as it passing through a small slit or any intricate surface. With Photography, this would apply to how the light interacts with the aperture and how it is detected by the photosites on a sensor. When light is focused through the aperture, it interacts with the edge of the aperture blades causing it to spread out and bend slightly, this causes a reduction with the maximum optical resolution. The smaller the aperture the greater the effect.
The depth of field calculator allows you to find the depth of field of any aperture of any lens at any distance. The depth of field calculator also includes a 35mm equivalency calculation so you can see equivalent lens crop and depth of a given aperture on smaller or larger sensor cameras compared to full frame 35mm.