The harpsichord was one of the preferred instruments to play the continuo part in baroque and early classical music. The other instrument was usually the organ. All keyboard instruments have complicated mechanisms that are operated by pushing a lever - the key.
The harpsichord is played by a small quill plucking the string when the key is pressed. After plucking the string, the quill falls away allowing it to vibrate. When the key is released, the quill returns to its starting point, and the damper stops the string's vibrations.