Polyphony refers to the number of notes that a digital piano can play simultaneously. 128-note polyphony means that the piano can play up to 128 notes at the same time, while 64-note polyphony means that the piano can only play up to 64 notes at the same time.
The more polyphony a digital piano has, the more realistic it will sound. This is because a piano with a higher polyphony can produce more complex and detailed sounds. For example, when you play a chord on a piano with a higher polyphony, you will hear each note of the chord clearly and distinctly. In contrast, when you play a chord on a piano with a lower polyphony, you may hear some notes drop out or sound muffled.
Additionally, a piano with a higher polyphony can handle more complex and advanced playing techniques. For example, pianists who play classical music or jazz often use techniques such as pedaling and glissando, which require the piano to be able to play multiple notes at the same time.
Overall, 128-note polyphony is generally considered to be better than 64-note polyphony for most pianists. If you are looking for a digital piano that will provide you with a realistic and expressive playing experience, then you should choose a piano with a higher polyphony.