Gold filled jewellery are jewellery that have been pressure bonding layers of real 14k gold on to a base metal such as brass. 
The 14k gold that’s on the outside of gold filled is the same gold as you would get from a solid 14k piece. So although a gold filled piece of jewelry is not solid gold, it has the properties and look of real gold. Resulting in a hypoallergenic, durable and anti-tarnish.