To answer the question I had to first read Hawthorne’s story. This quote is completely different when I read it in the context of the story.

First, the background. We know that “this crimson hand” is her hand-shaped birthmark. In context of the story, he is preparing to remove the birthmark that his otherwise beautiful wife has on her cheek using some chemical concoction that he created. This mark is so awful to him that it’s all he can think about. Although he had been assuring her that the procedure would not pose a risk, she had walked in on him preparing for it very nervous and talking with his assistant about this high risk that he had concealed from her. She assures him that he should tell her about this risk, and that it will not change her mind about having the procedure done. She hates the birthmark now more than he does. His quote is in response to her dialogue where she reassures him about her determination to rid herself and their relationship of the impact of this birthmark.

Sentence by sentence, here’s my interpretation of the quote. As with all literary commentary, this is only an interpretation.

“My noble wife,” said Aylmer, deeply moved, “I knew not the height and depth of your nature until now.

— He tells her that he had no idea that she would be so strong that she could know the true risk and still follow through with their plan.

“Nothing shall be concealed.”

— Now that he knows that she has such strong feelings, he feels more confident about telling her the truth.

“Know, then, that this crimson hand, superficial as it seems, has clutched its grasp into your being with a strength of which I had no previous conception. ”

— This little birthmark, even though it’s just on the skin of her cheek (superficial), has become such a part of his impression of her that he obsesses it, and he had previously no idea that it would be so strong.

In fact, in the end, it ends up being so strong that it causes him to risk giving his wife a potion so potent that it could kill her, which it eventually does.