Plated jewellery, as the name suggests, has a thin layer of precious metal over a base metal (ie zinc alloy, brass, stainless steel). The layer of precious metal is usually between 0.175 to 0.5 microns (in comparison, a dollar bill is 200 microns).
Since the precious metal layer is very thin, friction between your skin and the plated jewellery can slowly rub off the plating. Also, exposure to water, sweat, soap, perfume and other chemicals can cause the plating to fade.
