In William Shakespeare's "Sonnet 18," the line best paraphrases to but your youth will never fade.
In "Sonnet 18" Shakespeare tries to compare a fair maiden to a summer's day, but he expresses that there is nothing that compares to her because her perpetual beauty and youth are far greater than such a temporary, inconsistent season.
So, when Shakespeare writes "But thy eternal summer shall not fade," he is saying that her timeless youth will never fade, unlike the briefness of a summer's day.
