"Preparing" a piano involves placing objects (screws, rubber, bolts) on the strings to change the timbre.
Producers use the Skybox Audio library to manipulate these wave samples into unique "pieces" of music like cinematic scores or ambient tracks.
In music, "Waves" often refers to audio waveforms. There is a specific high-end virtual instrument called which features "prepared" piano sounds.
This is currently the most popular craft meaning for "preparing a piece." It involves using epoxy resin to create realistic-looking ocean scenes on wood or canvas.