In the intricately wrought Sonnet 12, Shakespeare weaves an underlying comparison, through metaphor, between the natural elements of time and the aging of humanity and decay of human beauty. A second metaphor compares annual time to a human life span. The sonnet is a lament of sorts that grows from the poetic speaker's contemplation of the signs of the passage of annual time and the juxtaposition of those signs with thoughts of the beloved beauty being addressed:
When I ... / see the brave day sunk in hideous night; / ... /
And sable curls all silver'd o'er with white; / ... /
Then of thy beauty do I question make,
That thou among the wastes of time must go,
The speaker carries the contemplation further, first, by saying that "Since sweets and beauties" of humanity fade and die even while a younger generation grows up,
Since sweets and beauties do themselves forsake
And die as fast as they see others grow;
and, second, by arriving at a conclusion. It is the conclusion to the speaker's contemplations that is represented in the lines that form the ending couplet of the Shakespearean (or English) sonnet:
And nothing 'gainst Time's scythe can make defence
Save breed, to brave him when he takes thee hence.
Recall that the underlying metaphor of the sonnet is the comparison of natural elements of time's passage to the life, beauty, old age and death of humanity, or, more specifically, a beloved human:
When I behold the violet [flower] past [its] prime,
And [your] sable [colored] curls all silver'd o'er with white; / ... /
And summer's green [fields] all girded up in [harvest] sheaves
Borne on the [funeral] bier with white and bristly beard [of old age], ….
The underlying metaphor is continued in the couplet wherein a natural sign of the passage of annual time again precedes a thought related to the life and death of the beloved. First, natural annual time passing is called up through the imagery of autumn's harvest when a scythe is used to harvest grain from the field; when it is harvest time, the year is almost over, and, according to the second metaphor, life is almost over.
The image is then made an additional, separate metaphor for universal time by giving time a persona and a possession: "Time's scythe." Time is said to be unstoppable through the statement that there is "no defense" against time's harvest, which is ultimate death: "And nothing 'gainst Time's scythe can make defence."
The last line of the couplet is the speaker's solution to the problem of the unstoppable march of time toward death: "That thou among the wastes of time must go." Usually, Shakespeare's solution to the decay of beauty and life is the immortalization of the beloved in poetry, and it might be said that he was quite successful in his efforts! In the final line of this sonnet, a different solution is offered, though it is a solution foreshadowed in the last line of the third quatrain: "And die as fast as they see others grow."
The speaker advises the beloved beauty to "breed," to bear children, so that when the scythe of Time comes, the beloved may put up a courageous front and face Time stalwartly ["brave"] as death approaches: "to brave him [Time] when he takes thee hence" into death. As a final note, the word "save" is used in its meaning that is synonymous with "except"; so "Save breed" means "except breed."