Piano-to-piano means that while connected to one another over the internet, two pianos can send and receive data to and from each other, allowing one piano to play the other, and vice versa. Instead of capturing the audio with a microphone and reproducing it with speakers like normal video chat, piano-to-piano sends data from one piano to the other that describes how it was played, so that the receiving piano perfectly replicates the playing of the sending piano.