A ditty is a short, simple song or a little poem you can almost hum after one hearing. People usually say it with a smile — “a little ditty” — meaning the thing is catchy and light, not serious concert music. You hear it of a folk tune, a children’s rhyme, or a silly song someone made up on the bus. It is slightly old-fashioned and almost always playful; younger speakers more often just say “a little song.” Though actually, in older English a ditty could also mean the words of a song, not only a cute short one. You would not use it for a national anthem or a long ballad you are treating with respect.