Castanets are concave pieces of wood that are used in certain styles of music.
An idiophone is any instrument made of a hard substance that you hit to get a sound. (not a drum-any drum is a membranaphone, with a skin stretched across it) Examples: bells, castenets, xylophones, gongs, steel pans, maracas, chimes and things like that.
The castenets are associated with Spain in most people's minds, but the attached Wikipedia article gives equal billing to many countries with Islamic origins or Spanish influence. If the question arises from a highschool music appreciation class, then "Spain" is the likely answer being looked for. If it is for a World Music Class, then it is worth reading the wikipedia article!