When jewelry or other items are identified as being gold filled, it means they are made from a hollow tube of usually 14K gold (sometimes 10K is used) that is filled with a base metal. The exterior of the gold filled item is solid karated gold and everything you can see or touch is a solid layer of karated gold.
This tube of karated gold is made by bonding the karated gold with the base metal using heat and pressure. With normal wear and tear and careful cleaning, the karated gold will never tarnish, chip or wear off. By law, the layer of karated gold has to be at least 1/20 of the total weight of the entire metal portion of the item.
Using gold filled material is much more expensive than using sterling silver or some type of base metal like copper, but it really cuts down on your cost of goods sold in comparison to using karated gold.