Sometimes a song does not begin on the first beat of a bar. Any notes which come before the first full bar are called lead-in notes (or pick-up notes). When lead-in notes are used, the last bar is also incomplete. The notes of the lead-in and the notes in the last bar add up to one full bar.
This 12 Bar Blues makes use of a lead-in note. Remember to use hand vibrato where indicated. In this example, a single hand movement is used on quarter notes while multiple hand movements are used on the longer notes. Once you are comfortable with this, try adding hand vibrato to other melodies you have learned.