Of course it is Shakespeare's Sonnet 55.
The sonnet shares its theme with that of several others, 18, 19, 65, 81, 107, 123, which oppose the power of verse to death and Time's cruel knife, and promise immortality to the beloved. Curiously enough, it does not seem to make any difference that the verse immortalizes the youth without revealing him, for the very fact of immortality seems to confer anonymity. The concluding couplet seems to be entirely satisfying, and we do not need to press further inquiries on the poet and demand to know who it is to whom eternal life is given. It is enough that he lives in lovers' eyes, for they comprehend all mysteries, and perhaps, on the last day, at the ending doom, we will know all the answers anyway, and realize that they were not all that important.