The number 925 stamped on a gold piece of jewelry usually means that the jewelry is not entirely gold; it is sterling silver that has been gold plated. The number normally indicates the purity of the silver used, not the quality of the gold.
Gold-plated jewelry often has other markings that can help determine whether a particular piece is gold plated. Common indications include the following: GEP, GP, HGE, HGP and RGP.
Jewelry that is sterling silver with a thin layer of gold plating is known as Vermeil. It is only worth a fraction of the value of pure gold.