What does this sonnet actually mean?

Question

Shall I compare thee to a summer's day?

Thou art more lovely and more temperate.

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 some time declines,

By chance, or nature's changing course, untrimm'd;

But thy eternal summer shall not fade,

Nor lose possession of that fair thou owest;

Nor shall Death brag thou wand'rest in his shade,

When in eternal lines to time thou grows't:

So long as men can breathe or eyes can see,

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

Answer 1

This sonnet is a love poem by Shakespeare.  He is looking at a youth and describing the beauty he sees there, both in complexion and character. He sees these qualities as immortal, even if he knows their transient nature.  Summer passes, but his vision of beauty lives on.

Answer 2

You are more beautiful than a summer's day, but the beauty of summer ends (summer's lease hath all too short a date).

Everybody gets old and their beauty fades (every fair from fair some time declines), but your beauty will never die (But thy eternal summer shall not fade) because it will live forever in this poem (so long lives this, and this gives life to thee).