A quaver is a trembling or shaking sound, especially in a person's voice. When you give your first speech in front of a crowd, don't be surprised to hear a quaver in your voice.
The nervous or emotional tremor in a person's speaking voice is one kind of quaver. It's also a primarily British term for an eighth note in music. To quaver can mean both to speak in a trembling voice, or to sing in a similar way, with a little warble or trill. The fifteenth century meaning of quaver is "to tremble or vibrate," and it's related to the earlier cwavien, "to shake," from the Low German word quabbeln, "tremble."