Before goint to secondary wavelet, first you have to know about wavefront.

A wavefront is the continuous locus of all such particles of the medium which are vibrating in the same phase at any time.

According to huygene principle, evey point on a wavefront acts as a fresh source of new disturbance,called secondary wavelets.

So, secondary wavelets or secondary waves are similar to the waves, the difference is only that the source of these waves are on the wavefront.