Melody is the notes we sing in a song or the notes that an instrument plays. Think of singing “Mary had a Little Lamb.” What you sing is the melody.
Harmony is the other notes that sound good with the melody. Sometimes we call the harmony “accompaniment.” Think of someone playing the music on the piano while you sing “Mary Had a Little Lamb.” What they play is the harmony.
Imagine that you know two sisters who are named “Melody” and “Harmony.” When they are playing together nicely, they like being together! When they are arguing or being mean, they don’t like each other so well. When the melody and the harmony in music match together well–usually in predictable intervals, like thirds, fourths, fifths, and sixths–it sounds so good! It’s like they are getting along. When the melody and harmony are in more disonant intervals, such as seconds or sevenths (and other intervals we haven’t named yet), we sometimes don’t like the way it sounds. Sometimes seconds and seventhssound good–it just depends on if the melody and harmony “resolve”–kind of like one person saying “I’m sorry” and the other person saying “That’s OK.”
Here are links to ideas for teaching melody and harmony: