Vibrato is a small, fast shaking of a musical note. The singer or player keeps the same note, but lets it move up and down in pitch just a little, again and again — maybe five or six times a second. You do not hear it as "wrong notes"; you hear it as a warm, alive, slightly trembling sound. A note with no vibrato sounds straight, flat, still — sometimes cold, sometimes very pure. That is why opera singers, violin players and cello players use a lot of vibrato: it makes a long note feel like it is breathing instead of just sitting there. On a violin the player makes it by rocking the left hand on the string; a singer makes it with the breath and the throat. The word is used in a technical way by musicians and teachers ('use more vibrato here', 'a wide vibrato', 'a fast vibrato'), and it can be countable or uncountable — you can say 'she has a beautiful vibrato' or 'sing it without vibrato'. Outside music people almost never use this word. Note that in singing, too much vibrato — very wide and very slow — is a fault; teachers call that a wobble. So vibrato is not simply 'good'; the right amount depends on the music. Old music (baroque) is often sung and played today with very little vibrato, because people believe that is closer to how it sounded then.