I'm going to assume that you mean Shakespeare's "Sonnet 18." That's the "Shall I compare thee to a summer's day" sonnet.
First, it is a Shakespearean sonnet. That means the poem is 14 lines long. It has an ABABCDCDEFEFGG rhyme scheme. Each line is 10 syllables written in iambic feet. That makes the entire sonnet iambic pentameter. Furthermore, because of the rhyme scheme, a Shakespearean sonnet is comprised of 3 quatrains and ends with a couplet.
The content of Sonnet 18 is the speaker's comparison of his lover to a summer's day. Summer is beautiful. Warm weather, trees with leaves, breezes, etc. Oh, and no school usually. Summer is amazing. The first line proposes the comparison between summer and the lover. The rest of the sonnet explains why the lover is more awesome and beautiful than summer. Line two says that she is more beautiful and "temperate". That means more stable and less extreme than say the warm day/cool night that summer has. Shakespeare also says that summer is entirely too short and not long lasting, unlike his lover. He says that her beauty will "never fade" and will outlast death itself.
That last set of lines explains how her beauty will defy the grave. Shakespeare more or less says that because he wrote it down, and men live and "breathe," they will be able to read this poem. She is forever immortalized within the sonnet. I guess Shakespeare was right.