The speaker remembers one gray November day spent quietly by a field, watching snow geese wheel and call so close that their wings seemed within reach. The moment felt complete. The speaker says they would have welcomed death then, because nothing more wonderful could be seen or wished for. That is a striking thing to say, and it shows how fully the sight filled them.
The present is bleaker. The trees are gone, the corn is plowed, and the speaker has "grown old with doubt." The geese no longer come. By calling them gods, the poem suggests that their arrival changed the very ground, and that their leaving left the air empty and the sky without white against the clouds. The mood is elegiac, a quiet mourning for something lost.
Then comes the last line, just three words: "They're back." It's abrupt and unadorned, and it lands like a gasp. After so much loss, the poem ends on surprise and renewed wonder.