A rattle was an object that made noise when shaken.
Rattles were sometimes employed in primitive medical care. In 2268, Spock sarcastically suggested that the android Norman was avoiding a physical from Doctor Leonard McCoy because he was terrified of the doctor's beads and rattles. (TOS: "I, Mudd")
Rattles were also used in a Klingon training discipline known as Hehh-duHpp. In 2374, Worf used a rattle while babysitting Kirayoshi O'Brien. Julian Bashir referred to the object as "baby's first bat'leth" and suggested that Worf leave it with the child, so Yoshi could practice. Later, Jadzia Dax reported that the child was indeed doing that very thing when she took him back to his family. (DS9: "Time's Orphan")