A polder is a piece of land below the surrounding water level. It is artificial because it is created by man. Dykes are build to make it dry and keep the water out, and canals are made to pump water out, dumping it in canals that eventually bring it back to sea. In the background you see a dyke: behind the dyke is water of a lake. If the dyke was not there you would look at the lake :-). So this is old and new land.