Incoming plane waves were reflecting from individual pebbles to produce a whole series of semi-circular ripples that spread back out into the water. You can see in the next two pictures that as the semi-circles expand, they coalesce to form another plane wave. This is Huygens' Construction. Huygens worked out what happened to a plane wavefront by breaking it down into point that sent out semi-circular ripples called secondary wavelets. He then worked out where the next wavefront would be by seeing where the semi-circular ripples coalesced. I have known the theory for years but had no idea that it was possible to see such perfect examples. Perhaps this is how he came up with the idea himself.
