Counterpoint is the art of composing music by combining different parts (voices) in a way that sounds nice. Music composed like this is called contrapuntal.
If the tune of Twinkle, twinkle little star is played on the piano and then some chords are added, this is harmony, not counterpoint. Instead, if the tune of Twinkle, twinkle little star was played while another melody was played, this would be considered counterpoint.