This is the concluding couplet of shakes peare's sonnet 18. The poet says that it is not appropriate to compare his friend with a summer's

 So long as men can breathe on eyes can see
So long lives this, and this gives life to thee

This is the concluding couplet of shakes peare's  sonnet 18. The poet says that it is not appropriate to compare his friend with a summer's day because his friend's beauty is more temperate and lovely than summer's day's beauty which is often marred by the vagaries of the weather. A Summer's day's beauty does not deserve to be compare with his friend's beauty because he will perpetuate his beauty in his poems. His friend's beauty will be made eternal.

The poet is convinced that his poems celebrating the beauty of his friend will endure until the end of time. What is interesting about the couplet is that the poet is not only celebrating hits friend's beauty, but he is also celebrating his own role as a poet in the perpetuation of mortal beauty. His friend's summer will be eternal in the sense that readers will read his poems for generations to come. 

The poet is  acutely conscious of the transience of youth and beauty. In the face of time's capacity for destruction it is possible to keep intact physical beauty. But poetry as an art form has comparitive permanence. In the earlier sonnets the poet requests his patron friend to marry and procreate so that his beauty may be perpetuated. But in this sonnet has gained the conviction that his friend's beauty can be made eternal in his poems.