Shakespeare's Sonnets

All of Shakespeare's sonnets follow the set "English form."  In each sonnet, there are fourteen lines total, divided into three groups of four lines (each proposing a different topic) and then a concluding couplet.  Your quote above is the concluding couplet of Sonnet 12.

This sonnet belongs to a group of Shakespeare's sonnets commonly referred to as the Procreation Sonnets.  These sonnets develop themes of mortality, death, the temporal nature of life, and point to being able to have children as the only means to continue to have one's presence felt on Earth.

Shakespeare usually addresses his sonnets to someone, and here, that someone is a young man in the full flower of his youth.  This young man is cautioned by Shakespeare to beware that he is destined for the same end (death) as all others, no matter how young and fair his now.

Since sweets and beauties do themselves forsake
And die as fast as they see others grow;

And nothing 'gainst Time's scythe can make defence
Save breed, to brave him when he takes thee hence.

Here is the couplet you cite and the preceding two lines, which remark upon the similarity of fate for all things (even sweets and beauties).

So, the simple meaning of this closing couplet is that "Time's scythe" (a nice image that recalls the Grim Reaper) cannot stand against the certain end for all living things (death).  Only "breed[ing]" (having offspring) can "brave" or stand against one's moment of death, or the moment that Time (Fate) chooses to "take thee hence."

For more on Sonnet 12, the structure of Shakespeare's Sonnets and the Procreation Sonnets, please follow the links below.