Aside from the answer you have already received, the poem is beautiful. It is a love poem which trips effortlessly off the tongue, and every single person on earth can relate to this subject. Love is always a popular topic for poetry and music. The speaker attempts to convince the one he loves to what extent he loves her...
Perhaps the most romantic lines in the poem are:
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.
Where the speaker basically says that her beauty will never fade, nor shall death claim her and their love since both will be eternally alive and youthful in the lines of this poem that many eyes will read even after they have physically left this world.