Vibrato can add feeling to a note - and surprisingly - keep it sustaining smoothly!
Here is a waveform of a note played with no vibrato, notice how shaky the wave
is.
The finger presses firmly down on the string between the frets. The hand moves side to side in line with the neck so that the fingertip 'rolls' on the string.
Here's a picture of the waveform with this type of vibrato:
Listen to a note played with vibrato 1
Using this vibrato requires pressing the finger tip down firmly on the string and quickly pulling the string down a little and then letting it straighten. This can be done by bending the finger a little or twisting the whole wrist.
Here's a picture of a waveform from this type of vibrato
Listen to a note played with vibrato 2