Those hours, that with gentle work did frame
The lovely gaze where every eye doth dwell
Will play the tyrants to the very same
And that unfair which fairly doth excel;
For never-resting time leads summer on
To hideous winter, and confounds him there,
Sap checked with frost, and lusty leaves quite gone,
Beauty o'er-snowed and bareness every where.
Then were not summer's distillation left
A liquid prisoner pent in walls of glass,
Beauty's effect with beauty were bereft,
Nor it, nor no remembrance what it was.
But flowers distilled, though they with winter meet,
Leese but their show; their substance still lives sweet.
He opens with mention of the passage of time – in hours. The first quatrain states the case that time has made the youth attractive, but it will continue its advance, and eventually make him "unfair" (meaning unattractive). In the next quatrain, he moves from talking of time in hours to time in seasons, where summer is related to the young man's current beauty, and winter to his eventual white hair and baldness (o'er-snowed and bareness) as well as his haggardness (sap checked with frost, and lusty leaves quite gone).
In the third quatrain, Shakespeare moves to talk about preserving summer by bottling perfume distilled from its flowers. Otherwise, he says, you'd forget about the beauty of summer, and have nothing to remember it by. The take-home message of this particular metaphor is that the young man had better reproduce, or else he'll die and leave nothing to his memory.
The closing couplet translates to something along the lines of "Flowers distilled into perfume lose their appearance, but their substance lives on." In the same way, I suppose, Shakespeare means that children would keep the memory and "substance" of the young man alive. What's remarkable to me is how Shakespeare shifts his metaphor in the closing couplet. Throughout the first three quatrains, he's using summer and winter to refer to the youth and his appearance and vitality; in the closing couplet, he shifts from the season (in which perfume is made), to focus on the flowers themselves (from which perfume is made) as being a metaphor for the youth.
If you're so inclined, compare and contrast this poem – you need to reproduce so that you live on – with, say, Sonnet 18 ("Shall I compare thee to a summer's day?"), which I included in April as part of my "Building a Poetry Collection" series for National Poetry Month, which concludes with these lovely lines:
So long as men can breathe, or eyes can see,
So long lives this, and this gives life to thee.
From Sonnet 18 on, the talk is no longer of children; it's the poet's tribute to the Fair Youth that will keep him alive in future generations.