Rough winds do shake the darling buds of May,

And summer's lease hath all too short a date: 
Sometime too hot the eye of heaven shines,
And often is his gold complexion dimm'd; 
And every fair from fair sometime declines,
By chance, or nature's changing course, untrimm'd;
But thy eternal summer shall not fade

Nor lose possession of that fair thou ow'st;
Nor shall Death brag thou wander'st in his shade,
When in eternal lines to time thou grow'st; 
So long as men can breathe or eyes can see,

So long lives this, and this gives life to thee. 

Shakespeare is thinking about time here. He's getting old, but the person he's writing to still has life ahead of him.

Capital-D-Death: Death personified. It's a very real thing in Shakespeare's eyes.