What does this quote from Shakespeare's Sonnet 12 mean?"And nothing 'gainst Time's scythe can make defence Save breed, to brave him when he takes...

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.