A zither is a musical instrument, kind of like a guitar without the neck and way more strings. You play a zither by strumming or plucking the strings with your fingers or a pick.
Zither comes from the same root word as guitar — the Greek word kithara for "lute.” They’re all stringed instruments, but a zither’s strings go across the sound box. An Autoharp is a kind of zither, so if you've played an Autoharp, you've played a zither. Zithers are often used in European folk music, and have about 30-40 strings. Originally, the zither was an Austrian instrument, but it’s been used in England since the mid-1850s.