A key focus of a number of Shakespeare's sonnets, and especially this one, is the way that the inevitable passing of time is going to rob his beloved (the person addressed in his sonnets) of his beauty. There are a number of solutions that are suggested, but here the speaker of this sonnet says that the only thing to do is for his beloved to have children, so that his beauty can "cheat" death and live on in the form of his progeny.
Bearing this overall summary in mind, let us consider some of the imagery to do with the rapaciousness of time and how it is depicted. The poem starts with the speaker looking at various natural sights and seeing how the passing of time effects them. The "brave day" is now "sunk in hideous night" and the violet is "past prime." Now, the "lofty trees" are actually "barren of leaves," and "summer's green" is all "girdled up in sheaves." The image that the speaker presents is one where nature's beauty in the fullness of summer has now passed and nature is "dying" with the onset of winter. This prompts the speaker of the poem to think of his beloved:
Then of thy beauty do I question make,
That thou among the wastes of time must go,
Since sweets and beauties do themselves forsake
And die as fast as they see others grow;
Unfortunately, the beloved must go "among the wastes of time" and decline and wither, just like the beauty of nature that the speaker has just described. Because of this, therefore, the poem ends with the only advice the speaker can give in this situation:
And nothing 'gainst Time's scythe can make defence
Save breed, to brave him when he takes thee hence.
Thus the imagery employed presents the passing of summer and the onset of winter with the inevitable "death" of nature, which allows the speaker to meditate on the ephemeral beauty of his beloved and what he must do as a result.