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.
The Bb note is the black key immediately to the left of the B note as shown in the diagram.
This Bb note is written in the space above the bass staff.
The C7 chord contains the Bbnote given above. To play the C7 chord, use the first, second and fifth fingers of your left hand, as shown in the C7 diagram.
When a flat sign is written before a B note, it applies to all B notes which come after the sign in that bar. Thus bar 3 of Exercise 75 contains four Bb notes.
To play this F chord, use the first, third and fifth fingers of your left hand, as shown in the F chord diagram.
The F chord shape intro duced on page 24 is the second inversion of this F chord.