The poet is observing swans while he feels “the sore heart”. Poet believes swans are the most perfect creatures that make his heart sore. “The sore of heart” can be described as a slight pain in the heart. When he sees swans moving around freely and carefree while he is tied to responsibilities and has a lot to worry about.
“The light tread” is symbolic of times when he walked freely nineteen autumns ago. He thinks about the better bygone days when he was not so burdened and walked freely like swans.