Polyphony in digital keyboards refers to how many notes it can play at one time. This covers both chords and arpeggios or fast runs: if one note starts before another ends, you need 2 voices to play them both.
Indeed, for keyboardists, mostly the ability to play really fast runs with the notes overlapping is most important. You only have 10 fingers, and even with both forearms on the keyboard, you're not likely to play more than 60 notes at once (even on an 88-key keyboard.) Some synthesists, though, use the midi inputs to the keyboard, setting different voicings to each input channel, and can rack up quite a lot of notes active at one time. I think this is more likely the way you'd notice the difference between 64 and 128 voice polyphony, but it is worth remembering that many of the songs recorded during the '60's and 70's were recorded on 4-track recorders: The more you layer up sounds, the less differentiation you get between one song and the next or one part of a song and the next.
And if you're just going to be playing it from the keyboard, don't let the number next to polyphony be your deciding factor, unless it's less than twice the number of fingers you have!