A chord is more than one note held down at the same time. On the piano that can be a combination of as little as 2 but is usually 3 notes held down at the same time. You use different fingers to hold down the keys depending on what chord you want to play. On a guitar that would be strings held down to form the chord. The combination makes the tone sound fuller than a single note played by itself. Look on Music Education at this site. I think I saw expamles and pictures given there for the basic chords people need to know and the fingers used to play them. Good luck.