A barrel-organ (sometimes known as a roller organ, or street organ) is a mechanical music instrument enclosed in a large, often highly-decorated, wooden box. In order to play it, cylinders with encoded music on them are turned by means of a handle.
The traditional name for a player of a barrel-organ is an organ grinder. Street organ grinders often had monkeys as an extra attraction to listeners; hence the phrase, 'speak to the organ grinder, not the monkey', which means speak to the person in charge, not to his/her minion.
Roses of Picardy (or sometimes Roses in Picardy) is a well-known First World War song, with lyrics by Frederick Weatherly, and music by Haydn Wood. It was popularized by Elsie Griffin, an opera singer who entertained British troops in France. German soldiers are said to have sung their own version of it.
Listen here to Roses of Picardy on Spotify.