Get out the microscope, because we’re going through this poem line-by-line.
One shade the more, one ray the less,
Had half impaired the nameless grace
Which waves in every raven tress,
Or softly lightens o'er her face;
- The balance between "shade" and light in the lady's beauty is so perfect that if you added one more "shade," or took away a single "ray" of light, you'd mess everything up.
- Fiddling with that balance at all would "half impair," or partially damage, the woman's beauty.
- Her beauty and "grace" are so hard to define that they're "nameless." The poet can't quite put his finger on what makes her so "grace[ful]," but he'll give it a try. After all, that's what the poem is doing – attempting to put sentiments into words.
- This "nameless grace" is visible in every lock of her black hair ("every raven tress") and it "lightens" her face.
- Look – more about the contrast of light and dark. The balance between light and dark that creates her "nameless grace" is apparent in both her dark hair and in the expression that "lightens" her face.
Where thoughts serenely sweet express
How pure, how dear their dwelling place.
- The expression on the woman's face shows how "serenely sweet" her "thoughts" are.
- Her "sweet" expression, the speaker reasons, is an accurate reflection of what's going on inside her mind, which is the "dwelling place" of her thoughts.
- Here we have another binary, or set of contrasts, to keep track of: her exterior expression, and her interior thoughts.
- The "sweet[ness]" of this lady's expression suggests that her mind is "pure" and innocent.
- "Dear," in this context (and in British English generally), means both precious and valuable.