It is a layer of gold, pressure bonded to another metal like brass. Gold filled is not the same as gold or gold plated as gold filled literally has more gold content in it. Gold-filled jewelry is much more valuable and tarnish resistant than gold plated.
We use gold-filled metals in our chains to prolong the life of your necklaces and claps as chains are usually exposed to sweat and chemicals from your skin care products more than other pieces.
Gold-filled is an affordable alternative to solid karat gold.