Here 'she' refers to the lady, Mrs. Wilmot Horton who is the wife of Lord Byron's cousin. Literally the title of the poem means when she walks, she is looking beautiful.

               The beauty of the lady is compared with the night sky, which is cloudless and full of stars - "She walks in beauty, like the night / Of cloudless climes and starry skies." 'Best of dark' suggests the darkness which is free from any gloominess. And 'best of bright' suggests the brightness without any dazzling colour. This combination of darkness and brightness is shown in the face and eyes of that particular lady. The mingling of darkness and brightness is the unique beauty of the lady. This combined colour of gloomiless dark and brightless bright male the beauty of the lady comfortable or 'tender'.  Here 'gaudy day' indicates that day which is excessive bright or dazzling. But the poet here says by the name of heaven that the beauty of the lady is much more beautiful than the 'gaudy day'. 

               The poet says that the beauty of the lady is so much perfect that if one adds a dot of darkness or one decreases a little bit of brightness, then the present beauty of the lady must be spoiled - "One shade the more, one ray the less/ Had half impair'd the nameless grace." Here the poet uses the word 'nameless' because he does not give her name in the poem and addresses her by the pronoun 'she'. Here 'raven' is the bird with dark colour. Here it is used to suggest the dark colour of her hair. The poet also says that all the area of her hair is black in colour. The glow of her face softly lightens over her face. He means that the glow makes her much beautiful. Serene and sweets thoughts are expressed through the beauty of the lady. The poet asserts that the 'dwelling place'from where these thoughts are aroused is the mind which must be beautiful also - "Where thoughts serenely sweet express / How pure, how dear their dwelling-place." 

                The poet says that has cheek and 'brow' (forehead) are also soft, calm and revelatory or expressive - "And on that cheek and o'er that brow / So soft, so calm, yet eloquent....." And the smile of that lady can easily win the heart of anyone. 'Tints' suggest the blush of the lady's face. These smiles, tint cheek and brow reflect three aspects of her life -

            (i) Virtuous past life

            (ii) Peaceful mind

            (iii) Innocent love of her heart.