This is a funny way of describing someone – what does it mean to "walk in beauty," anyway? Is "beauty" some kind of cloud or vapor that the woman is walking in? Or is it like a comfortable pair of shoes?
And all that's best of dark and bright Meet in her aspect and her eyes: (lines 3-4)
This woman's beauty takes the "best" things from both ends of the spectrum ("dark" and "bright"). And it's not just that she has a pretty face – her whole "aspect," or appearance, is harmonious and lovely.
Thus mellow'd to that tender light Which heaven to gaudy day denies. (lines 5-6)
The balance of "dark" and "bright" in the woman's appearance sort of averages out into a "tender light." It's like she casts off a glow, but a soft one, or something.
One shade the more, one ray the less, (line 7)
This line is perfectly balanced, just like the woman's beauty. There's a repeated structure, divided by a comma in the middle of the line – the comma is like a pivot point. You've got two sets of opposites ("shade" and "ray," "more" and "less") that are balanced on either side of that comma.
Which waves in every raven tress, (line 9)
No wonder he's comparing the woman to "night" instead of to "a summer's day" – this lady is a brunette, so it makes sense to compare her darker coloring to "night."
The smiles that win, the tints that glow, (line 15)
Her "smiles" and "tints," or blushes, come and go – obviously they're not permanently etched on her face. And that dynamism, or changeability, is part of what makes her so beautiful.