Diffraction is an optical effect which can limit the total resolution of your photography– no matter how many megapixels your camera may have. Ordinarily light travels in straight lines through uniform air, however it begins to disperse or “diffract” when squeezed through a small hole (such as your camera’s aperture). This effect is normally negligible, but increases for very small apertures.
Since photographers pursuing better sharpness use smaller apertures to achieve a greater depth of field, at some aperture the softening effects of diffraction offset any gain in sharpness due to better depth of field.
