Answer:
A harp is a stringed instrument which has its strings anchored in the soundboard. Each string is tuned to a different note of the scale, somewhat like a piano.
Unlike the piano, however, most harps have only seven strings in each octave. That would limit the harp to only one key (without retuning), if it did not have a mechanism to independently change the length of the strings.
Pedal harps have metal pins mounted on rotating disks, which grab the string at the right points to shorten the vibrating length of each string, raising the pitch by one or two semitones. The disks are rotated by a system of rods connected to the pedals in the base of the harp, so that the player can change key or play accidentals (sharps and flats) on the fly.