This is a flat sign.
A flat sign written before a note on the staff means that you play the note that is one fret lower than the note written. For example, the notes written on the following staff are both called B flat (Bb). The first Bb is played on the first fret of the 5th string, and the second Bb is played on the third fret of the 2nd string. When a flat sign is written on the staff, it is always written before a note.