Someone's "tone of voice" means how they sound. It especially means the emotional characteristics or personality in someone's voice. For example, if your son or daughter starts to speak to you angrily, you can say:
Don't you take that tone of voice with me!
Or if someone says that they're OK, but they say it in a sad and depressed voice, you can describe it this way:
I could tell from his tone of voice that he was upset.