“It tells you the truth. As for how to read it, you'll have to learn by yourself.”

The Master and Lyra, in Ch. 4 : The Alethiometer
His Dark Materials, The Golden Compass (1995)
Context: "Lyra, I'm going to give you something, and you must promise to keep it private. Will you swear to that?"
"Yes," Lyra said.
He crossed to the desk and took from a drawer a small package wrapped in black velvet. When he unfolded the cloth, Lyra saw something like a large watch or a small clock: a thick disk of gold and crystal. It might have been a compass or something of the sort.
"What is it?" she said.
"It's an alethiometer. It's one of only six that were ever made. Lyra, I urge you again: keep it private. It would be better if Mrs. Coulter didn't know about it. Your uncle — "
"But what does it do?"
"It tells you the truth. As for how to read it, you'll have to learn by yourself. Now go — it's getting lighter — hurry back to your room before anyone sees you."