What does Then in a wailful choir the small gnats mourn Among the river sallows borne aloft Or sinking as the light wind lives or dies mean?

This line from John Keats' poem "To Autumn" describes the mournful sound of small gnats near a river. The gnats are depicted as singing in a sorrowful choir as they are carried by the wind, rising and falling with its movements. This imagery captures the melancholic beauty of the autumn season.