The frequencies produced by an instrument or voice create its timbre. Because each instrument or voice vibrates slightly differently, each produces different frequencies. This is how our ears can tell one from another.
What determines the timbre of an instrument?
Timbre is determined by an instrument's shape (e.g., the conical or cylindrical pipe of a wind instrument), by the frequency range within which the instrument can produce overtones, and by the envelope of the instrument's sound.
What does it mean to have timbre in your voice?
Timbre in modern English generally refers to the quality of a sound made by a particular voice or musical instrument; timbre is useful in being distinct from pitch, intensity, and loudness as a descriptor of sound.