Need with this question what is a zither?

A Zither is an instrument.

It has a flat wooden box and strings. You place it horizontally and with your fingers and a plectrum you pluck the strings.
A plectrum is a thin flat piece of plastic, that you either wear or hold with your fingers. You use it to pluck strings of an instrument. 

Hope that helps!
-Chris

It’s an instrument that you use to pluck an instrument