Diffraction is the spreading out of waves as they pass through an aperture or around objects. It occurs when the size of the aperture or obstacle is of the same order of magnitude as the wavelength of the incident wave. For very small aperture sizes, the vast majority of the wave is blocked. For large apertures the wave passes by or through the obstacle without any significant diffraction.
In an aperture with width smaller than the wavelength, the wave transmitted through the aperture spreads all the way round and behaves like a point source of waves (they spread out below).
This is shown in the diagram below:
Figure 1: Single slit diffraction when a wave passes through an aperture with width smaller than the wavelength (d, is less than, lambda,d<λ). For a significant amount of the wave to pass through, the aperture must be close to the size of the wavelength.