According to Huygen's principle, each point on a wavefront is a source of wavelets which generates the next wavefront.
In the given diagram, point source of light that is the primary source is represented by W. X and Z are wavefronts. The points marked on X are point source of wavelets. On Z, a wavelet is represented by Y. The wavelets spread out in forward direction. The envelope of wavelets forms a wavefront.
