When I consider every thing that growes
Holds in perfection but a little moment,
That this huge stage presenteth nought but showes
Whereon the Stars in secret influence comment;
When I perceive that men as plants increase,
Cheared and checkt even by the selfe-same skie;
Vaunt in their youthfull sap, at height decrease,
And were their brave state out of memory;
Then the conceit of this inconstant stay,
Sets you most rich in youth before my sight,
Where wastfull time debateth with decay
To change your day of youth to sullied night,
And all in war with Time for love of you
As he takes from you, I ingraft you new.
Changes to the original text: full stop changed to semicolon at the end of line 4 and line 8
In the first quatrain, the poet notes that everything that grows holds its perfection for only a brief moment, that the world is but a huge stage on which things appear, and events happen under the influence of the stars.
In the second quatrain, the poet observes that men and plants exist under the same sky, which acts to make them prosper (chear, vaunt) or decline (check, decrease). Line 8 'were' is 'wear'. The meaning of the line is that things or men fallen from their prime only have a memory of their perfection (brave state), but other meanings are possible, which you may like to canvas, dear reader.
In the third quatrain, the poet sets up a situation (conceit) in which he postulates that the young man (rich in youth) is threatened by time and decay, personified as two sinister characters having a debate about how best to bring about his downfall. The verse is almost a mise en scène.
The final couplet expresses the idea that the poet himself has taken up the fight against time, busily 'ingrafting' new life where it is destroyed, thus defeating time's purpose. This could mean simply that the poet writes about the young man.