Often considered to be a great budget-friendly alternative to karat gold, gold filled jewelry is made of a layer of solid gold (which is much thicker than that used in standard gold plated pieces) that layered over a base metal like brass, copper, or sterling silver. As a general rule, gold must make up at least 5% of the total weight of the piece in order to be gold filled pieces is usually sterling silver.
It's interesting to note that the process of gold filling was actually invented in 1817 by a man named John Turner and was incorporated into many Victorian-era jewelry pieces.
Aside from the fact that it's less expensive, gold filled jewelry also has other lesser-known advantages compared to karat gold. For instance, it's more durable and can last a decade, or perhaps even up to 30 years, depending on the karat amount.
Sources:
https://www.gemsociety.org/article/jewelry-metals-overview/
https://weddingknowhow.com/jewelry/gold-filled-jewelry-faq/#:~:text=How%20is%20G
old%20Filled%20jewelry%20marked%3F%20When%20purchasing,to%20avoid%20confusion%20with%20other%20types%20of%20gold.
Pictured Above: Gold Filled Lacey Open Work Filgree Drop Earrings by To the Nines Jewelry