This is another way of representing the amount of pure gold used to make the piece of jewellery. This is equivalent to 9ct.
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.
37.5 indicates it is 37.5% of pure gold. It means the same as 9ct. Yellow gold has other additives such as Silver, Copper and Zinc.
This metal is not as malleable as 750 or 18ct gold and is mainly used in casting pieces of jewellery