This is some sentences above it:
Riddle stepped closer.
“It’s all over,” he said. “I’m going to have to turn you in, Rubeus. They’re talking about closing Hogwarts if the attacks don’t stop.”
“‘N at d’yeh —”
Please provide us with more information. Tell us the chapter, who is talking what they are doing and the sentences before and after. That's called context and it's a requirement of the forum.
I think there's a mistake in the text you are using.
It should be "What d'yeh ... " which means "What do you?", written as it sounds when spoken.