Do you mean Shakespeare's Sonnet 18?
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 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 owest;
Nor shall Death brag thou wander'st in his shade,
When in eternal lines to time thou growest:
So long as men can breathe or eyes can see,
So long lives this and this gives life to thee.
This sonnet is one of Shakespeare's most famous ones because it is pretty easy to understand. It is a metaphor for his love or, as some have suggested, a dear friend. The "love" or "friend" is the summer day. The sonnet points out all the parts of a beautiful summer day that remind him of his love/friend.
The summer day has some negative aspects, however, whereas his love does not. His love "is more beautiful than a summer day" - and he explains why. His love is more temperate; sometimes a summer day can get too hot. Sometimes summer winds "shake" away the beautiful May buds. Finally, unlike the summer day, which has an end, the poet says that his love for his friend will live forever because this love is immortalized by the poet's verse.
There are some cool metaphors: the eye of heaven is the sun. "The eternal lines" are the poet's verse, etc. The sonnet has a rhyme scheme of abab, cdcd, efef, gg - very regular. It is written in iambic pentameter and the couplet at the end sums up the theme, that as long as there are people on earth to read the poem, his love will live on.