Pedal (music)
There are seven pedals on a concert harp.  Each pedal can be in a high, middle or low position.  The strings of the harp can be thought of like the white notes of a piano, tuned to naturals (C,D,E etc.).  Each pedal can change one of the letter-named notes one semitone higher or lower. For example: the pedal on the left changes all the Ds to D flat (if the pedal is in the highest position) or D sharp (in the lowest position).  From left to right the pedals operate the Ds, Cs, Bs, Es, Fs, Gs and As.