Maximum polyphony means how many notes can sound on the piano at the same time.
This includes not only the total number of notes you are playing on the keys with your fingers, but also the notes that are still sustaining while you press the damper pedal. For this reason, you may hear notes being cut off on some models if the total number of notes that are actually sounding exceeds the unit’s maximum polyphony. For complicated songs in which many notes are played or in which the pedal is frequently used, we recommend that you use a model that features unlimited polyphony.
For the maximum polyphony of each model, check the “Specs” tab on the Web page of the product in question.