Ans The line is taken from 'sonnet no 18: Shall I Compare Thee to a Summer's Day?’
William Shakespeare is the poet.
Here ‘this’ refers to the sonnet of Shakespeare.
The poet compares the beauty of his beloved friend to the beauty of Summer. The poet believes that every lovely natural object is changed or decayed but his friend’s beauty. The poet thinks that his poetry is immortal and through this sonnet the beauty of his friend will remain immortal. Death and time will not be able to snatch his beauty for, as long as men will breathe and eyes will see, his verse will be read and these eternal lines will immortalize the beauty of his friend. In this way the poet's friend will get life and live forever.