Or softly lightens o'er her face; (line 10)
Byron has said that the woman's whole "aspect" is beautiful, but now he's trying to put his finger on exactly what makes her facial expression so gorgeous.
And on that cheek, and o'er that brow, (line 13)
Now Byron's attempting to break down her beauty, one piece at a time, in order to understand it. It's like he's dissecting her beauty.
The smiles that win, the tints that glow, (line 15)
The woman's smiles are "win[ning]," and apparently she blushes a lot – is he trying to suggest that she's a bit of a flirt?