Answer: By comparing humans to clouds.
In this passage, Shelley compares humans to clouds in order to emphasize how we are constantly moving and changing. He tells us that clouds "restlessly... speed, gleam, and quiver." This describes the movement of the clouds. He also tells us that in the end, night closes around them, and they are lost forever. This also describes how souls, after all their movement and changes, inevitably stop existing, and are lost forever.