Vibrato is an effect where the fretted note is moved slightly in and out of pitch by "bending" the string. To do this push the string upwards then pull it downwards as shown below.
Do this motion continuously to create a vibrato effect. The speed of the vibrato depends on the pitch of the note e.g. the vibrato of a low note is played slower than that of a high note. Vibrato is also useful for sustaining notes.
Experiment with this new technique on various different notes and listen very carefully to the sound. It may take a while before you feel comfortable playing vibrato and make it sound natural.
The next lick uses a vibrato on the D note of the 2nd bar.