9ct is a measure of the amount of pure gold used to make the piece of jewellery.
Pure (fine) gold is measured as 24ct and represents 100% gold. Pure gold is very soft so is unsuitable for jewellery as it would mark easily, the claws holding gemstones would bend easily as well.
9ct represents 9/24 or 37.5% of pure gold. Yellow gold has other additives such as Silver, Copper and Zinc.
This metal is not as malleable as 18ct and is mainly used in casting pieces of jewellery.
There is also 9ct White Gold which has additives such as Nickel, Copper and Zinc. These metals retain the white colour of the gold. 9ct Rose Gold was often used in antique jewellery and became popular again 20 years ago. It has more Copper and less Silver than Yellow Gold to enhance the pink colour.