What's he saying?
"That time of year thou mayst in me behold / When yellow leaves, or none, or few, do hang"
You may see in me the autumn of my life, like the time when yellow leaves, or no leaves, or a few leaves still hang
"Upon those boughs which shake against the cold / Bare ruin'd choirs, where late the sweet birds sang."
(The leaves hang on) branches, which shiver in anticipation of the cold; the branches are like empty, ruined church choir pews, and sweet birds used to sing on the branches.
"In me thou seest the twilight of such day / As after sunset fadeth in the west,"
You see in me the twilight of my life, like when the sunset has faded to darkness in the west,
"Which by and by black night doth take away / Death's second self, that seals up all in rest."
Which before long is replaced by the black night, Death's second self, which covers everything in a deathly sleep.
"In me thou see'st the glowing of such fire / That on the ashes of his youth doth lie,"
You see in me the glowing of a fire that is burning atop the ashes of its earlier burning (my youth),
"As the death-bed whereon it must expire / Consumed with that which it was nourish'd by."
The ashes are now the death-bed upon which the fire will go out, consumed by the very thing it was nourished by before.
"This thou perceivest, which makes thy love more strong / To love that well which thou must leave ere long."
Because you see this, your love is made stronger, to love well that which you must soon leave.
Why is he saying it?
Sonnet 73 is almost as exemplary as sonnet 60 in expressing the theme of the ravages of time. The sonnet focuses on the narrator's own anxiety over growing old and, like sonnet 60, each quatrain of sonnet 73 takes up the theme in a unique way, comparing the narrator's "time of year" (i.e., stage of life) with various examples of the passing of time in nature. The metaphors shorten in duration from months to hours to what may be minutes, the acceleration itself a metaphor for the increasingly rapid rate at which old age begins to take its toll on the human body.
In the first quatrain, the narrator compares himself to the late autumn season, that time of year when the trees have begun to lose their leaves and the cold is setting in. Some scholars suggest that this metaphor was deliberately chosen for its imagery of barrenness where there once was growth, a possible allusion to Shakespeare's incipient baldness. Quatrain two makes life still shorter, going from the seasons of the year to the hours of the day. The narrator is at the twilight of his life: his sun has set, and Death is soon upon him.
But even so, the emptiness of death is not fully established until quatrain three, where it is finally understood by the narrator as something permanent. Whereas the changing of the seasons and the passing of day and night occur in (presumably) infinite cycles, a fire is not reborn from its ashes, and its extinguishment means the end. Time is the enemy; Time is Death. The passing of time is the creator and the destroyer of life.
With that said, the closing couplet of sonnet 73 is like an admonition: one's love should grow stronger as one's time left to love is running out. It is not entirely clear whether this line is addressed specifically to the fair lord or in fact to himself, or perhaps even to both, since the narrator's approaching death will mean that each must bid the other farewell. In any case, the narrator is clearly distressed by his inevitable fate: old age, death, and eternal separation from the fair lord.
A great number of parallels can be drawn between the imagery of sonnet 73 and that of the other sonnets, which makes this an interesting example of the consistency of Shakespeare's symbolism and figurative language. The passing of the seasons was encountered in sonnet 18: "And summer's lease hath all too short a date." We also saw the sun as a metaphor for human life in sonnet 60, although there we followed its development from birth to maturity whereas here in quatrain two it has already begun to die. Finally, the image in the third quatrain of a fire being "Consumed with that which it was nourish'd by" reminds us of a line from sonnet 1: "Feed'st thy light'st flame with self-substantial fuel." Note the two instances of color symbolism in sonnet 73, also with referents in other sonnets: yellow is used in sonnets 17 and 104 as the color of age or passing time, while black is used repeatedly throughout the sonnets to symbolize the "other," that which is sinful or dreaded.