They were standing on the edge of a huge chessboard, behind the black chessmen, which were all taller than they were and carved from what looked like black stone. Facing them, way across the chamber, were the white pieces. Harry, Ron and Hermione shivered slightly –– the towering white chessmen had no faces.
"Now what do we do?" Harry whispered.
"It's obvious, isn't it?" said Ron. "We've got to play our way across the room."
Behind the white pieces they could see another door.
"How?" said Hermione nervously.
"I think," said Ron, "we're going to have to be chessmen."
He walked up to a black knight and put his hand out to touch the knight's horse. At once, the stone sprang to life. The horse pawed the ground and the knight turned his helmeted head to look down at Ron.
"Do we –– er –– have to join you to get across?"
The black knight nodded.
(Harry Potter and the Sorcerer's Stone)
Is what the same as in What are those things on the table?, or does the clause have some other meanings? For example, what is not a pronoun but an adverb.