This is a flat sign.
A flat sign written before a note on the staff means that you play the note that sounds one fret lower than the note written. For example, the note written on the staff above is called B flat (Bb), and is played on the third fret of the third string. When a flat sign is written on the staff, it is always written before a note.
Strum Pattern
As with the sharp sign, instead of writing a flat sign before every Bb note on the staff, it is easier to write just one flat sign after the treble clef. This means that all the B notes on the staff are played as Bb, even though there is no flat sign placed before them.
Strum Pattern
This song is another round. The second player begins when the first player reaches bar 9.