When a wave passes through a barrier with a hole of diameter comparable to the wavelength, the hole acts approximately like a point source. If the barrier contains two holes, the waves from the point sources may interfere, having high amplitude (dense dots) where crests align with crests and where troughs align with troughs, and having low amplitude (sparse dots) where crests nearly align with troughs. The resulting wave varies qualitatively depending on the separation of the holes.
In quantum mechanics, individual photons (for example) may be approximated by plane waves. After encountering a barrier with two holes, an individual photon can interfere with itself. Macroscopically, this can manifest as diffraction of light off a grating, or a pattern of faint but regularly-spaced bright spots behind a screen with two holes.
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