Byron wrote this after seeing a cousin in mourning dress. The praise is all about balance — darkness and light, measured exactly — which is a strange and rather clever way to flatter someone.
She walks in beauty, like the night Of cloudless climes and starry skies; And all that's best of dark and bright Meet in her aspect and her eyes;
Underlined words and phrases can be tapped for a note
She moves through beauty the way a clear night full of stars does, and everything best about light and dark meet in her face and her eyes.
One shade the more, one ray the less, Had half impair'd the nameless Which waves in every raven tress, Or softly lightens o'er her face;
Underlined words and phrases can be tapped for a note
One shade more or one ray less would have half-ruined the nameless grace that moves in every dark curl of her hair and softly lights her face.
Metre, metaphor, and the sentence inside the line. Every answer comes with the reason and the line that proves it.
That's the whole thing.
Read it again slowly — it gets better the second time.