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, as follows.
To play the note Bb play the black key immediately to the left of the B note as shown in the diagram.
Instead of writing a flat sign before every B note on the staff, it is easier to write just one flat sign after each clef. This means that all the B notes on the staff are played as Bb, even though there is no flat sign written before them.