Vibrato is a way of sustaining a note once it has been picked. It also gives much more expression to your playing. Vibrato is usually found on long notes, or a note at the end of a phrase. Blues vibrato is usually obtained by the movement of the wrist away from the neck and back towards the neck, much like turning a doorknob. The symbol for vibrato is a wavy line.
The next lick uses vibrato on the last note of the phrase.