Why does Claude Monet's work Waterlilly Pond relate to Impressionism?

Because Claude Monet is world renounced for being one of the leading founders of the style of art called 'Impressionism' which relates to the way that light reflects of everyday subject matter and the colours that the light evokes.

He is well known for his series of paintings of lilies, hay stacks, and his wife Camille.

If you need to know more, Wikipedia has tonnes of information on Claude Monet.