This is another way of representing the amount of pure gold used to make the piece of jewellery. This is equivalent to 18ct.
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.
750 indicates the piece of jewellery is 75% of pure gold. Yellow gold has other additives such as Silver, Copper and Zinc.