What is the metaphor concerning shadows of branches on the concrete?

From the text:

Gus motioned down to the shadows of the branches intersecting and coming apart on the concrete. “Beautiful, huh?”
“Yeah,” I said.
“Such a good metaphor,” he mumbled.
“Is it now?” I asked.
“The negative image of things blown together and then blown apart,” he said. Before us, hundreds of people passed, jogging and biking and Rollerblading. Amsterdam was a city designed for movement and activity, a city that would rather not travel by car, and so inevitably I felt excluded from it. But God, was it beautiful, the creek carving a path around the huge tree, a heron standing still at the water’s edge, searching for a breakfast amid the millions of elm petals floating in the water