When I am finishing a picture, I hold some God-made object up to it - a rock, a flower, the branch of a tree or my hand - as a final test. If the painting stands up beside a thing man cannot make, the painting is authentic. If there's a clash between the two, it's bad art.

This quote is saying that when an artist is finished with a painting, they should compare it to something made by nature. If the painting looks as good or better than something from nature, then it is a good painting. But if the painting looks worse than something from nature, then the painting is not a good one.