I think I know how a harp works now.

There are seven notes to an octave in each key. The harp has seven pedals, one for each note in a seven note scale. Each pedal has three positions. When all the pedals are in the middle position, the harp is in C major. (Or A minor.) If you want to play in F major, you would find the pedal that controls the B strings, and make all the Bs flat a half step. If you want to play in G major, starting with all the pedals in the middle position, you would find the pedal that controls the F strings, and make it sharp a half step. Continue in that manner for each key.
It struck me last night that the harp is a forerunner to the pedal steel guitar.