The speaker asks the person he is addressing to look at him as a representation of late autumn, moving into an image of skeletal winter trees compared to destroyed monastery ruins, conveying a sense of fading life and approaching death.
That time of year thou mayst in me behold
You can look at me and see a stage of life that corresponds to a specific season.
When yellow leaves, or none, or few, do hang
That season is late autumn, represented by a few yellow leaves clinging to branches, or almost none left at all.
Upon those boughs which shake against the cold,
Those branches tremble in the cold air, just as an aging body shivers.
Bare ruin'd choirs, where late the sweet birds sang.
Those empty, freezing branches are compared to the roofless ruins of church choir lofts where sweet-singing birds recently performed.