A half-pedal is a feature on some keyboards that allows the player to sustain a note for less than the full duration by pressing the pedal only partway down. This can be useful for creating more subtle or nuanced sustain, or for imitating the sound of acoustic instruments that don't sustain indefinitely when a key is held down. Not all keyboards have this feature, so if it is something you are interested in, be sure to check for it before making your purchase.