The term 'Harmless Rattler' refers to a type of maraca, which is a percussion instrument consisting of a hollow gourd filled with small pebbles or seeds. The maraca is shaken to produce a rattling sound, and the Harmless Rattler is a type of maraca that produces a softer, less intense sound than other types of maracas. It is often used in Latin American music, as well as in some forms of jazz and rock music.