It means that there are 3 beats per measure and one quarter note equals one beat.
the Mona lisa is a half-length portrait of Lisa Gherardini by the Italian Renaissance artist Leonardo da Vinci. the painting is a portrait of Lisa Gherardini, the wife of Francesco del Giocondo and is believed to have been painted between 1503 and 1506. Leonardo may have continued working on it as late as 1517.
