Gold filled is a metal composed of a solid layer of gold that is bonded to a core base metal through heat and pressure. Gold filled jewelry will not wear off over time, and if taken care of properly it can last a lifetime. Gold fill is indistinguishable from solid gold but costs significantly less because of its base metal core.
It differs from gold plate in that gold plate consists of a thin, microscopic layer of gold that will wear off over time. The gold on gold filled is around 15 times thicker than gold plated jewelry.
Unlike gold plate, it's safe to get your gold filled jewelry wet. You can wear it to bed, in the shower, and while you exercise. It's safe to swim in gold filled (though we generally suggest avoiding pools and hot tubs for any type of jewelry).