Scholars believe that William Shakespeare wrote his 154 sonnets between 1592 and 1598. It's generally accepted that Sonnets 1-126 are addressed to a young man known as the "Fair Youth." In Sonnets 1-17, the poet-speaker urges the young man to marry and to have beautiful children in order to ensure his mortality.

Look in thy glass, and tell the face thou viewest
Now is the time that face should form another;
... But if thou live, remember'd not to be,
Die single, and thine image dies with thee. (Sonnet 3)

In Sonnet 18 the speaker changes his approach, stops emphatically urging the Fair Youth to "form another," and explores the idea that the Fair Youth achieves immortality through the speaker's poems.

Sonnet 18 is one of the least affected of all the sonnets, which is one of the reasons why it speaks so clearly and directly to us. The language is simple and straightforward, and there are few literary devices used in the poem. Every line appears to be self-contained, as a single thought contributing to the entirety of the poem.

The last six lines of the sonnet contain the essence of the poet's argument, that as long as his poems exist and are read, the Fair Youth is immortal.

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. (Sonnet 18)

The Fair Youth's defense against time is the poet's poem.

Interestingly, the poet emphasizes this idea in the next sonnet. In Sonnet 19, the poet directly addresses Time, warning Time to do whatever it likes, but to do nothing to change the Fair Youth.

Devouring time, blunt thou the Lion’s paws,
And make the earth devour her own sweet brood,
Pluck the keen teeth from the fierce Tiger’s jaws,
And burn the long-lived Phoenix in her blood;
Make glad and sorry seasons as thou fleets,
And do whate’er thou wilt, swift-footed time,
To the wide world and all her fading sweets:
But I forbid thee one most heinous crime,
O, carve not with thy hours my love’s fair brow,
Nor draw no lines there with thine antique pen;
Him in thy course untainted do allow,
For beauty’s pattern to succeeding men.

The last two lines of the sonnet are almost mocking of Time's power to impose its will, and mortality, on everything in the world. No matter what Time does, the poet says, the Fair Youth will live forever, and live forever young, in his poetry.

Yet, do thy worst, old Time: despite thy wrong,
My love shall in my verse ever live young. (Sonnet 19)