I'm the sort of person who loves to read Shakespeare aloud on a lazy afternoon, so here's a friendly, line-by-line modern take on 'Sonnet 18' that I like to share when someone asks what the poem actually says.
1. "Shall I compare thee to a summer's day?" — Should I compare you to a summer day?
2. "Thou art more lovely and more temperate:" — You're more beautiful and more steady/mild than one.
3. "Rough winds do shake the darling buds of May," — Strong winds can batter May's precious flower buds,
4. "And summer's lease hath all too short a date;" — and summer's time is far too short;
5. "Sometime too hot the eye of heaven shines," — Sometimes the sun gets too hot,
6. "And often is his gold complexion dimm'd;" — and its golden face can get clouded;
7. "And every fair from fair sometime declines," — Everything beautiful eventually loses its beauty,
8. "By chance or nature's changing course untrimm'd;" — whether by accident or simply by nature's changes.
9. "But thy eternal summer shall not fade," — But your own long-lasting summer won't die away,
10. "Nor lose possession of that fair thou ow'st;" — you won't lose the beauty you possess;
11. "Nor shall Death brag thou wander'st in his shade," — Death won't be able to boast that you've gone into his shadow,
12. "When in eternal lines to time thou grow'st:" — because you grow into time through these eternal lines (these verses);
13. "So long as men can breathe or eyes can see," — As long as people are alive and can see,
14. "So long lives this and this gives life to thee." — these lines live on, and they keep you alive.
Reading it out like this always makes me smile — Shakespeare basically argues that the poem itself is the immortality machine. I usually end up reciting it to friends at coffee shops, and people are always surprised how direct his point actually is.