This is a flat sign. When a flat sign is placed before a note on the staff, it means that you play the key immediately to its left. This key may be either black or white. The note B flat (written as Bb) is shown on the staff below.
This Bb note (above Middle C) is the black key immediately to the left of the B note, as shown in the diagram.
This Bb note is written on the third line of the treble staff.