Vibrato is a very common device used by violinists which causes the pitch of a note to vary up and down quickly. This is achieved by moving the finger pressing on the string slightly forwards and backwards. Vibrato is often perceived to create a more emotional sound, and it is employed heavily in music of the Romantic era. There are several different styles of vibrato ranging from the use of just the fingers, to the use of the wrist or even the whole forearm. These produce different effects and are favoured by different players for different styles of music. Some styles of music use little or no vibrato at all.
It is often thought that vibrato can partially disguise an out of tune note, the intuitive idea being that the ear should not be able track pitch as accurately when it is moving up and down. However, recent experimental work finds no such effect: the human ear detects the mean frequency of a vibrato note just as accurately as it detects a non-undulating pitch. It is not necessarily the case that results obtained under careful experimental conditions will carry over to real-life playing, and there is at least some evidence that vibrato may be able to disguise mistuning at faster tempos. Nevertheless, it now appears that individuals learning to play the violin are well advised never to suppose that using vibrato will help them with their pitch problems.
>> Watch a video demonstrating vibrato