Vibrato is a technique in which one or all of the fingers on the left hand shakes, or vibrates, to produce a higher quality of sound. To perform this technique correctly requires practice, as most things to when learning to play an instrument. Physically, what is done when vibrato is used is a simple process; however, to understand its mechanics and to perform it effectively are two different things.
Vibrato is performed by, when playing a note, pressing against the neck with the involved finger and the thumb, and then pushing the hand upwards and downwards about the neck.
The thumb and finger should remain pressed firmly against the fingerboard and back of the neck while this is performed. Essentially, as you wobble your finger back and forth, slightly moving it up and down along the string, the sound will als "wobble", becoming slightly higher and lower as your finger moves accordingly. The sound will have a slightly oscillating quality, and resonates better than a flat pitch.
When practicing vibrato, a student should generally try to keep oscillations as even as possible. One way to do this is to try to relax the arm as much as possible, loosening the arm to prevent tightened muscles from "overdoing" any one forward or backward pulse.
The majority of cellists, and string instrumentalists, use this technique. Vibrato can be used on any note, although cellists are constantly disputing over how many notes to use vibrato on. Some think that all notes should have vibrato, while some argue that vibrato should only be used for emphasis on a few notes.
However, regardless of their ideas of how much vibrato to use, almost all cello teachers teach students how to do vibrato, in addition to guidelines on how to use it. Vibrato is, once again, a technique that requires much practice to perfect, and even more to know how to use it appropriately. This guide should be used as a physical reference: your own personalized vibrato technique may vary slightly, as each player's style is different.