Shakespeare's sonnets always have a rhymed couplet at the end. It is his poetic practice.

SHALL I COMPARE THEE TO A SUMMER'S DAY

Bring out the meaning of the concluding couplet of the poem 'Sonnet no. 18' ?

→ Shakespeare's sonnets always have a rhymed couplet at the end. It is his poetic practice. The present sonnet is naturally not an exception to this. The rhymed couplet forms a separate stanza. Here the poet sums up beautifully the arguments put forward in the quatrains. The poet states clearly that his friend's beauty is eternal.

He logically  supports his point in the rest of the poem. But in the couplet he concludes more strongly that he makes such assertive statement not without reason. Here he means to suggest that his poetry will be appreciated by the posterity. And as his poetry glorifies the beauty of his friend, he will enjoy an endless lease of life.