This song introduces first and second endings (see line 6). On the first time through, ending one is played, as indicated by the bracket: Z The section of music is then repeated (go back to the beginning of line 5) and on the second time ending two is played. Be careful not to play both endings together.
If you are playing the chords to this song it is easier to play the Gb7, F7 and B chords as bar chords. To learn how to play bar chords see Progressive Guitar Method: Bar Chords.