18ct 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.
18ct represents 18/24 or 75% of pure gold. Yellow gold has other additives such as Silver, Copper and Zinc. This purity of gold and the additives keeps it malleable, that is allows it to bend but is stronger so keeps its shape. It also doesn’t mark so easily. Yellow Gold displays a lovely warm gold colour.
There is also 18ct White Gold which has additives such as Nickel, Copper and Zinc. These metals retain the white colour of the gold. 18ct 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.