The following lines appear in "The Rape of the Lock" Canto Five, lines 25-26:
But since, alas! frail Beauty must decay,
Curl'd or uncurl'd, since Locks will turn to grey,
In Canto Five, the action of the poem turns to the consideration regarding the beauty of women and how society looks at the beauty of women.
This being said, at the end of the poem and Canto Five the recognition regarding ones ability to sustain immortality comes into question. The lines above refer to the fact that only in Heaven can a person and , or more importantly, the locks sustain their glory. The lines, therefore, refer to the fact that on earth beauty diminishes (including the golden color of the locks changing to grey). Only in Heaven, in the stars, can Belinda and her locks remain beautiful and admired for their beauty in a timeless way.
Simply, the lines refer to the fact that both beauty and all things associated with the beauty will decay.