This is a flat sign. When a flat sign appears before a note, it lowers the pitch of the note by one semitone or one half step.
For more information about tones and semitones, see Progressive Theory by Peter Gelling. The following diagram shows the fingering for the note B flat (Bb). Each time you see a flat sign in front of the note B, you will need to play this fingering.
A flat sign is always placed before the note.
Like the sharp sign, the flat sign affects all B notes within the bar in which it appears. The flat sign is also cancelled by a bar line, meaning that a new flat sign is needed to indicate a new Bb note in the following bar.